author | Peter Fordham <peter.fordham@gmail.com> |
Mon, 22 Mar 2010 12:54:15 -0700 | |
branch | CompilerCompatibility |
changeset 11 | 8d1ef0d13f16 |
parent 0 | e4d67989cc36 |
permissions | -rw-r--r-- |
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/** |
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@mainpage |
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@par Overview |
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@par |
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This document describes new core APIs for semi-automatic resource |
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management designed to begin to increase the baseline usability of |
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Symbian OS for developers. We would welcome feedback, improvement |
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suggestions, and defect reports. Note that there is a Q&A section at |
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the bottom of this page that explains some of the background rationale |
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behind the design choices we have made to get to this point. |
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Please send any questions or review feedback you have to: core.idioms@symbian.com |
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The new APIs include the following: |
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|
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- A new type category: the L-class |
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- Allows leaving constructors and can rely on automatic destructor execution for |
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cleanup (explained in more detail below) |
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- New general purpose, self-managing string classes that can be used much |
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like T-classes |
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- LString16 |
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- For manipulating strings of 16 bit characters |
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- Usually used via the ::LString typedef |
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- LString8 |
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- For manipulating strings of 8 bit characters |
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- ::LData |
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- Type alias for LString8 allowing the intent to use it as a raw (non-character) data buffer to be expressed |
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- The LString classes are based on the descriptor APIs and are fully interoperable with descriptors |
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- The LString classes can grow on demand and do not always need to be |
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initialized with a fixed reserve maximum length |
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- New "smart pointer" style utility classes for managing resources referenced from local variables, |
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which may be used instead of the existing cleanup stack API |
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- LCleanedupPtr |
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- Alternative to PushL(CBase*)/Pop()/PopAndDestroy() code sequences |
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- LCleanedupHandle |
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- Alternative to CleanupClosePushL()/Pop()/PopAndDestroy() code sequences |
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- See also the more rarely used LCleanedupRef, LCleanedupArray, and LCleanedupGuard |
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- New "smart pointer" style utility classes for managing resources referenced from data members, |
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which reduce the need for manually-written destructors |
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- LManagedPtr |
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- Alternative to calling delete manually in a destructor for the managed data member |
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- LManagedHandle |
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- Alternative to calling Close() manually in a destructor for the managed data member |
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- See also the more rarely used LManagedRef, LManagedArray, and LManagedGuard |
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- Enablers for the (optional, typically private) use of single-phase construction |
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- #CONSTRUCTORS_MAY_LEAVE |
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- A class declaration flagging that the class's constructors may leave |
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- Normally used in combination with the LManagedXxx classes to ensure reliable cleanup in case of a constructor leaving |
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- Normally hidden behind a conventional Symbian NewL() abstraction |
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- New utilities for making correct, error checking code easier to read |
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- New #OR_LEAVE postfix macro |
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- Alternative to wrapping User::LeaveIfError() around an expression |
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The motivation for the new APIs is to make it easier to write correct, |
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less brittle code in the first instance, but perhaps more so to make |
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the resulting code easier to read and easier to modify safely later. |
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Some guiding principles: |
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- Resource management should be handled either fully automatically or declaratively |
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at point of definition as far as possible |
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- Helps to eliminate mismatches between code that pushes and code that pops |
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- Helps to eliminate mismatches between code that constructs and code that destructs |
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- Removes the need to duplicate cleanup code (or modify your code's structure) to correctly handle functions with multiple return points |
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- Makes changing code safer and easier by reducing dependencies between different areas of the code |
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- It should be easier and lighter weight to write and maintain leave-safe code |
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- So that more people are prepared to make the investment to do it right |
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- The primary function of code should not be swamped by auxiliary error checking and resource management |
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- The key logic should be front and centre, and read as straightforwardly as possible |
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- It should be easier to write fully general code |
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- Unsafe and/or overly conservative maximum size limitations should not be |
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encouraged by making that style of code easier to write |
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- Terminology should be as conventional as possible |
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- We call a string a string |
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To help illustrate the new APIs, here is a sampling of idiomatic code |
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implemented first in terms of the current Symbian APIs, and alongside |
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in terms of the new APIs. |
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<table width="100%"> |
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<tr valign="top"> |
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<td>Current APIs</td><td>New APIs</td> |
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</tr> |
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<tr valign="top"> |
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<td> |
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@code |
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{ |
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TBuf<KMaxQuery> query; // fixed worst-case max |
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query.Format(KQueryFormat, param); |
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ExecuteQueryL(query); |
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} |
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@endcode |
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</td> |
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<td> |
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@code |
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{ |
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LString query; // can grow its heap buffer on demand |
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103 |
query.FormatL(KQueryFormat, param); |
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104 |
ExecuteQueryL(query); |
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105 |
} // query buffer released on normal scope exit or leave |
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106 |
@endcode |
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107 |
</td> |
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108 |
</tr> |
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109 |
|
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110 |
<tr valign="top"> |
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111 |
<td> |
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112 |
@code |
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113 |
{ |
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HBufC* queryc = HBufC::NewLC(KTooBigForStackMaxQuery); |
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TPtr query(queryc->Des()); |
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BuildQueryL(query); |
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ExecuteQueryL(query); |
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CleanupStack::PopAndDestroy(); |
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} |
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120 |
@endcode |
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121 |
</td> |
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122 |
<td> |
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123 |
@code |
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124 |
{ |
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LString query; |
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126 |
BuildQueryL(query); |
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ExecuteQueryL(query); |
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} |
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129 |
@endcode |
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130 |
</td> |
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131 |
</tr> |
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132 |
|
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133 |
<tr valign="top"> |
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134 |
<td> |
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135 |
@code |
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136 |
{ |
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RBuf query; |
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query.CleanupClosePushL(); |
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query.CreateL(TooBigForStackMaxQuery); |
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140 |
BuildQueryL(query); |
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141 |
ExecuteQueryL(query); |
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142 |
CleanupStack::PopAndDestroy(); |
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143 |
} |
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144 |
@endcode |
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145 |
</td> |
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146 |
<td> |
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147 |
@code |
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148 |
{ |
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149 |
LString query; |
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150 |
BuildQueryL(query); |
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151 |
ExecuteQueryL(query); |
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} |
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153 |
@endcode |
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154 |
</td> |
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155 |
</tr> |
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156 |
|
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157 |
<tr valign="top"> |
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158 |
<td> |
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159 |
|
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160 |
@code |
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161 |
{ |
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162 |
CQuery* query = CQuery::NewL(); |
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163 |
CleanupStack::PushL(query); |
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query->BuildQueryL(); |
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query->ExecuteQueryL(); |
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166 |
CleanupStack::PopAndDestroy(); |
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167 |
} |
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168 |
@endcode |
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169 |
|
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170 |
</td> |
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171 |
<td> |
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|
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173 |
@code |
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174 |
{ |
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175 |
LCleanedupPtr<CQuery> query(CQuery::NewL()); |
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176 |
query->BuildQueryL(); |
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query->ExecuteQueryL(); |
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} // query deleted on normal scope exit or leave |
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179 |
@endcode |
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180 |
|
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181 |
</td> |
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182 |
</tr> |
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183 |
|
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184 |
<tr valign="top"> |
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185 |
<td> |
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186 |
|
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187 |
@code |
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188 |
{ |
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CQuery* query = CQuery::NewL(); |
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190 |
CleanupStack::PushL(query); |
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query->BuildQueryL(); |
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CleanupStack::Pop(); |
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return query; |
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194 |
} |
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195 |
@endcode |
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196 |
|
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197 |
</td> |
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198 |
<td> |
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199 |
|
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200 |
@code |
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201 |
{ |
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202 |
LCleanedupPtr<CQuery> query(CQuery::NewL()); |
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query->BuildQueryL(); |
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return query.Unmanage(); |
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205 |
// query was protected until Unmanage() was called |
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206 |
} |
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207 |
@endcode |
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208 |
|
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209 |
</td> |
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210 |
</tr> |
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211 |
|
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<tr valign="top"> |
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<td> |
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@code |
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{ |
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RQuery query; |
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CleanupClosePushL(query); |
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query.BuildQueryL(); |
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query.ExecuteQueryL(); |
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CleanupStack::PopAndDestroy(); |
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} |
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@endcode |
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</td> |
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<td> |
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@code |
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{ |
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LCleanedupHandle<RQuery> query; |
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query->BuildQueryL(); |
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query->ExecuteQueryL(); |
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} // query is closed on normal scope exit or leave |
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@endcode |
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</td> |
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</tr> |
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</table> |
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@par Concepts |
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These APIs introduce a new category of Symbian type: the |
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L-class. L-classes have characteristics close to standard C++ value |
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and handle classes, including the following: |
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- Constructors, operators, and implicit operations may leave |
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- They are self-managing and do not require auxiliary memory management logic |
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L-classes may be used like T-classes except that they can own |
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resources, they typically rely on guaranteed execution of their |
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destructors, and they cannot necessarily be naively bitwise |
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copied. Any code that manipulates L-class instances must be |
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leave-safe. The only exception to this rule is if the particular |
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operations being used are explicitly documented not to leave (e.g. the |
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default LString constructor and all the LManagedXXX constructors). |
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The L prefix denotes that these classes may be Liberal when it comes |
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to Leaving, while being Leave-safe themselves. |
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The implementation of many L classes relies on the leave=throw mapping |
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current in Symbian OS since v9.1 because this guarantees destructor |
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execution for objects allocated on the stack. |
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@par Notes on Usage |
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For ease of experimentation during review, all the new APIs are |
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packaged for use as a statically-linked bundle: |
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- #include <euserhl.h> in your code |
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- STATICLIBRARY euserhl.lib in your mmp |
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Some important dos and don'ts (most of which leavescan will reliably flag): |
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- The LCleanedupXXX classes are for managing locals only, and may not be used to manage data members. This is because the LCleanedupXXX classes are implemented in terms of the legacy cleanup stack. |
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- The LCleanedupXXX classes must not be used in the same function as the legacy cleanup stack APIs. They can interact unintuitively and this is best avoided. It is all or nothing at function granularity. |
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- Use L-suffix method variants with LCleanedupXXX, never LC-suffix variants that manipulate the cleanup stack. This is for the same reason as the previous point. |
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- The LManagedXXX classes should only be used for data members. If they are used for locals, they can interact unintuitively with cleanup stack based cleanup (from other functions on the call stack) with respect to cleanup ordering. |
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- Whenever you write code that manipulates LString or other L-prefix classes, always conservatively leave-safe your code. This is now easier with the LCleanedupXXX utilities. |
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On the resource management utility classes: |
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- A managed object's member functions may be called directly on the managing container, but -> notation must be used to access them |
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- Implicit coercions from managing containers to managed types are not defined; instead the dereference operator * must be used e.g. to obtain a reference suitable for passing as a parameter to another function |
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- Pop()-like semantics (as opposed to PopAndDestroy()) is achieved by calling Unmanage() on the managing container before it goes out of scope; Unmanage() returns the managed object which can then be returned from your function, or passed to a different function which takes over ownership. Unlike Pop(), however, Unmanage() simply sets a flag to disable cleanup and may be called at any time without worrying about stack discipline |
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- It is still typically necessary to declare, define, and export a destructor, even if its implementation (currently) contains no hand-written code |
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- The destructor must be available wherever delete may be called on an instance of the class |
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- The compiler will automatically generate destruction logic behind the scenes for LString and LManagedXxx data members, even if you don't write any code yourself |
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- The destructor may of course change over time anyway |
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On the LString classes: |
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- Existing library functions that accept a modifiable TDes cannot take advantage of LString's on-demand growth behaviour; the necessary capacity must first be explicitly reserved in the LString before calling such functions (this can be done either with a constructor parameter or by calling SetMaxLengthL()) |
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- Any descriptor method that could panic due to lack of capacity is replaced with a leaving variant that grows the heap e.g. instead of Append(), LString defines AppendL(). The non-leaving TDes versions are hidden to avoid accidental use, although they can still be accessed via a TDes typed (as opposed to LString typed) ref or pointer |
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- Programmers new to Symbian OS could first be introduced to descriptors and string handling via just const TDesC& (for input parameter contracts), TDes (for output parameter contracts), and LString for implementation. LStrings can be initialized by value from any descriptor return type, and by automatically taking ownership in the HBufC* return case. |
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On single-phase construction: |
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|
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- This option is enabled by LString and the LManagedXxx classes |
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- However you implement construction behind the scenes, NewL is an important abstraction |
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- Think hard before using single-phase construction in a framework class designed for derivation in client code |
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- LString and the LManagedXxx classes may be used safely in conventional Symbian two-phase construction as well as standard C++ style single-phase construction |
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- LManagedXxx construction never leaves, and is safe to perform at any stage |
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- Default construction of an empty LString never leaves, but other initializations must be deferred to ConstructL |
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- An understanding of standard C++ construction semantics is required before attempting to use single-phase construction |
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- If a constructor throws mid-way through, its matching destructor is not invoked to clean up |
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- Instead all data members fully-constructed to the point of the leave are automatically destructed |
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- Therefore, in effect, all class cleanup must typically be achieved via data member destruction |
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- The presence of a non-empty destructor is a warning sign if you are using single-phase construction |
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An annotated example using the new APIs follows (the |
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euserhl_walkthrough example goes into much more detail than we do |
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here). |
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@code |
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#include <e32std.h> |
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#include <f32file.h> |
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#include <euserhl.h> // Includes all the new APIs |
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class CFinder : public CBase |
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{ |
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public: |
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// We have opted to use single-phase construction here, and some of |
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// our constructor's initialization actions may leave. In order to |
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// guarantee full cleanup in all cases, we have to declare this fact. |
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CONSTRUCTORS_MAY_LEAVE |
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static CFinder* NewL(const TDesC& aPattern); |
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~CFinder(); |
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void GetNextMatchL(TDes& aMatch); |
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|
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protected: |
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CFinder(const TDesC& aPattern); |
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|
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protected: |
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LString iPattern; |
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LManagedHandle<RFs> iFs; // We always use LManagedXxx for data members |
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// ... |
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}; |
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|
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CFinder* CFinder::NewL(const TDesC& aPattern) |
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{ |
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return new(ELeave) CFinder(aPattern); |
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} |
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|
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CFinder::CFinder(const TDesC& aPattern) |
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// This initializer may leave, since the LString will allocate a |
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// heap buffer large enough to contain a copy of aPattern's data |
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: iPattern(aPattern) |
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{ |
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// If connection fails and we leave here, iPattern's destructor |
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// will be called automatically, and the string's resources will |
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// be released |
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iFs->Connect() OR_LEAVE; |
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} |
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|
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CFinder::~CFinder() |
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{ |
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// Automatic destruction of each of the data members does all of the |
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// work for us: iPattern's heap buffer if freed, while Close() is |
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// called on the managed RFs in iFs. |
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|
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// Even though this destructor is textually empty, it should |
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// still be exported; the compiler is generating destruction logic |
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// for us in this case |
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} |
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|
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void CFinder::GetNextMatchL(TDes& aMatch) |
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{ |
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// ... |
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LString possible; |
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// ... |
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LCleanedupHandle<RFile> file; // We always use CleanedupXXX for locals |
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TInt status = file->Open(*iFs, possible, EFileRead); // Note use of -> and * here |
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// ... |
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aMatch = possible; |
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// ... |
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} |
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@endcode |
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|
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For comparison, the original code in terms of existing APIs: |
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|
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@code |
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#include <e32std.h> |
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#include <f32file.h> |
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|
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class CFinder : public CBase |
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{ |
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public: |
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static CFinder* NewL(const TDesC& aPattern); |
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~CFinder(); |
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void GetNextMatchL(TDes& aMatch); |
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|
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protected: |
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CFinder(); |
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void ConstructL(const TDesC& aPattern); |
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|
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protected: |
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HBufC* iPattern; |
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RFs iFs; |
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// ... |
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}; |
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|
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CFinder* CFinder::NewL(const TDesC& aPattern) |
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{ |
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CFinder* self = new(ELeave) CFinder; |
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CleanupStack::PushL(self); |
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self->ConstructL(aPattern); |
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CleanupStack::Pop(); |
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return self; |
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} |
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|
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CFinder::CFinder() |
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{ |
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} |
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|
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void CFinder::ConstructL(const TDesC& aPattern) |
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{ |
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|
418 |
iPattern = aPattern.AllocL(); |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
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|
419 |
User::LeaveIfError(iFs.Connect()); |
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parents:
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|
420 |
} |
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parents:
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|
421 |
|
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parents:
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|
422 |
CFinder::~CFinder() |
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Revision: 201002
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parents:
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|
423 |
{ |
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parents:
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|
424 |
iFs.Close(); |
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parents:
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|
425 |
delete iPattern; |
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parents:
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|
426 |
} |
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parents:
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|
427 |
|
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
428 |
void CFinder::GetNextMatchL(TDes& aMatch) |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
429 |
{ |
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parents:
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|
430 |
// ... |
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parents:
diff
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|
431 |
RFile file; |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
432 |
CleanupClosePushL(file); |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
433 |
TInt status = file.Open(iFs, possible, EFileRead); |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
434 |
// ... |
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parents:
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|
435 |
aMatch = possible; |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
436 |
// ... |
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parents:
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|
437 |
CleanupStack::PopAndDestroy(); |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
438 |
} |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
439 |
@endcode |
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parents:
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|
440 |
|
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parents:
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|
441 |
@par Q&A |
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parents:
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|
442 |
|
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Revision: 201002
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parents:
diff
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|
443 |
\b General.1 <em>Why start at the bottom? Shouldn't improving the usability of Symbian |
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parents:
diff
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444 |
OS and its APIs be a top-down initiative?</em> |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
445 |
|
e4d67989cc36
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
446 |
There is a product-wide usability initiative within Symbian which aims |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
447 |
to address all of APIs, documentation, and tools. On the API side, |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
448 |
there are tracks that try to improve the design process for good API |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
449 |
usability going forward, and tracks like this one that try to improve |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
450 |
the usability of existing functionality. This is just the first |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
451 |
deliverable of the latter effort, but other work is ongoing. |
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parents:
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|
452 |
|
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
453 |
\b General.2 <em>Will Core Idioms changes like these be backed up by tools support?</em> |
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parents:
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|
454 |
|
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parents:
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|
455 |
Yes, support for tools like Leavescan and Coverity, as well as more |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
456 |
basic debugger presentation support for the new descriptor types and |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
457 |
resource management classes, is being planned into the roll-out |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
458 |
project. |
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parents:
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|
459 |
|
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parents:
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|
460 |
\b General.3 <em>Will Core Idioms changes like these be backed up by matching |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
461 |
documentation, examples, and coding standards updates?</em> |
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Revision: 201002
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parents:
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|
462 |
|
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Revision: 201002
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parents:
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|
463 |
Yes, we understand this to be essential and this is our goal. |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
464 |
|
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Revision: 201002
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parents:
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|
465 |
\b General.4 <em>Symbian will support STL as part of Open Environment, and |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
466 |
Symbian's STL implementation includes std::string and |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
467 |
std::auto_ptr. Why not just use them?</em> |
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parents:
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|
468 |
|
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Revision: 201002
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parents:
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|
469 |
We investigated this option to the point of creating a native Symbian |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
470 |
C++ interoperable STL subset for experimentation. This was ultimately |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
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|
471 |
rejected for the following reasons: |
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|
472 |
|
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|
473 |
- Hybrid-style code (with mixed Symbian-style and standard-style naming) is |
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parents:
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|
474 |
harder to read and to work with than code in a single consistent style |
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|
475 |
- Beyond basic naming conventions, supporting two very different APIs |
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parents:
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|
476 |
for manipulating strings (the descriptor methods alongside the |
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Revision: 201002
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parents:
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|
477 |
std::string methods) similarly does not help usability |
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Revision: 201002
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parents:
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|
478 |
- Interoperability between std::string and TDesC/TDes accepting APIs |
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Revision: 201002
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parents:
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|
479 |
implemented via implicit coercion is more limited than the direct placement |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
480 |
of new string classes within the existing descriptor hierarchy can |
e4d67989cc36
Revision: 201002
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parents:
diff
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|
481 |
achieve |
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Revision: 201002
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parents:
diff
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|
482 |
- Because we need a number of Symbian-specific resource management |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
483 |
utilities (cleanup-stack aware, R-class aware, and so on), auto_ptr |
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Revision: 201002
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parents:
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|
484 |
on its own does not provide much leverage |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
485 |
- It would require maintaining two subtly variant STL builds: the Open |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
486 |
Environment version with standard C++ new and exception |
e4d67989cc36
Revision: 201002
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parents:
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|
487 |
behaviour, and a Symbian C++ version with new(ELeave) and leaving |
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Revision: 201002
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parents:
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|
488 |
behaviour |
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parents:
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|
489 |
|
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Revision: 201002
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parents:
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|
490 |
Instead we opted to give Symbian C++ a "programming philosophy |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
491 |
upgrade"; more aligned with standard C++ but still in the style of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
492 |
Symbian C++ and its existing APIs. The result is something that can be |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
493 |
more easily and more incrementally adopted by existing Symbian C++ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
494 |
projects and teams. Note that this decision to focus first on building |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
495 |
out the usability of native Symbian C++ does not preclude the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
496 |
possibility of an official hybridized Symbian embedding of STL being |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
497 |
developed at a later date |
e4d67989cc36
Revision: 201002
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parents:
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|
498 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
499 |
Finally, we could have used Symbian-ized names like XAutoPtrYyy as the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
500 |
basis for the resource management utility classes. However, this was |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
501 |
deliberately avoided because the semantics of our classes are |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
502 |
different from auto_ptr e.g. they can't support copy construction or |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
503 |
transfer of ownership through parameter passing and return, and |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
504 |
different variants are required for use with locals than with data |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
505 |
members. We did not want to give the Symbian classes names that could |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
506 |
result in developers making natural but incorrect assumptions about |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
507 |
their behaviour based experience with similarly-named classes |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
508 |
elsewhere. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
509 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
510 |
\b General.5 <em>These new template classes seem more conceptually |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
511 |
complex than what they're replacing. How can this improve |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
512 |
usability?</em> |
e4d67989cc36
Revision: 201002
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parents:
diff
changeset
|
513 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
514 |
Template classes can be complex to implement, but they're not |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
515 |
particularly difficult to understand or use in the way they're |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
516 |
employed here. Symbian programmers are already familiar with the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
517 |
basics from RArray<T> and RPointerArray<T>. And to anyone coming from |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
518 |
standard C++ and STL it's second nature, as in fact is this general |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
519 |
style of parameterised declaration to desktop Java and C# programmers |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
520 |
used to generics. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
521 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
522 |
In the end, it's just a concise, declarative annotation that takes |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
523 |
leaky code and helps turn it into robust code. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
524 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
525 |
Leaky: |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
526 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
527 |
@code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
528 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
529 |
CFoo* foo = CFoo::NewL(); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
530 |
foo->ExecuteL(); // the CFoo leaks if this leaves |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
531 |
if (foo->IsAsync()) |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
532 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
533 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
534 |
return; // the CFoo leaks if we return |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
535 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
536 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
537 |
} // the CFoo leaks if we exit scope normally |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
538 |
@endcode |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
539 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
540 |
Safe (new style): |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
541 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
542 |
@code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
543 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
544 |
LCleanedupPtr<CFoo> foo(CFoo::NewL()); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
545 |
foo->ExecuteL(); // the CFoo is automatically deleted if we leave |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
546 |
if (foo->IsAsync()) |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
547 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
548 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
549 |
return; // the CFoo is automatically deleted if we return |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
550 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
551 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
552 |
} // the CFoo is automatically deleted if we exit scope normally |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
553 |
@endcode |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
554 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
555 |
Safe (classic style): |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
556 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
557 |
@code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
558 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
559 |
CFoo* foo = CFoo::NewL(); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
560 |
CleanupStack::PushL(foo); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
561 |
foo->ExecuteL(); // the CFoo is automatically deleted if we leave |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
562 |
if (foo->IsAsync()) |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
563 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
564 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
565 |
CleanupStack::PopAndDestroy(foo); // the CFoo is deleted manually |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
566 |
return; |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
567 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
568 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
569 |
CleanupStack::PopAndDestroy(foo); // the CFoo is deleted manually |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
570 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
571 |
@endcode |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
572 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
573 |
\b General.6 <em>Don't some of these APIs simply trade depth for width? There are less lines of code but the statements are longer and more complex</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
574 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
575 |
Placing as much information at point of declaration as possible guards |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
576 |
against separate lines of code becoming inconsistent with one-another, |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
577 |
particularly over time in the face of later code changes. Where |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
578 |
cleanup code is duplicated, for example in the case of a function with |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
579 |
multiple exit points, or where the cleanup point is separated by tens |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
580 |
of lines of code from the creation point, these risks increase |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
581 |
further. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
582 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
583 |
Another way to look at this is that it is far easier in a real, |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
584 |
non-trivial piece of code to take an unprotected local (a raw pointer |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
585 |
to an object) and to apply cleanup protection to it (for example if a |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
586 |
call to a leaving function has been added) once at its point of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
587 |
creation, than it is to study what might be a quite complex flow of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
588 |
control in order to determine where all the matching calls to |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
589 |
CleanupStack::PopAndDestroy() need to go. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
590 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
591 |
In some cases, faced with the CleanupStack, you might actually choose |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
592 |
to rework an existing function to have a single exit point to simplify |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
593 |
the process of leave-protecting its code. The risks inherent in that |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
594 |
are not necessary using the mode of protection offered by these new |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
595 |
APIs. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
596 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
597 |
\b General.7 <em>These APIs seem to come with a lot of new rules, |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
598 |
particularly when it comes to interoperating with existing |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
599 |
functionality. If it turns out people need to understand both the new |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
600 |
and old idioms in order to mix them safely, where's the benefit? |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
601 |
Doesn't it actually make things worse?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
602 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
603 |
The interoperability constraints are far from ideal, but we believe |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
604 |
they are manageable and that the benefits outweigh the issues, |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
605 |
particularly so for developers working on new code that uses these new |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
606 |
APIs and idioms consistently. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
607 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
608 |
Wherever possible sanity checking will be offered in code analysis |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
609 |
tools like Leavescan and/or in UDEB runtime checking via assertions. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
610 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
611 |
\b Cleanedup.1 <em>The LCleanedupXxx/LManagedXxx distinction is confusing. Why can't we |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
612 |
just use the same classes everywhere?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
613 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
614 |
We would much prefer to be able to use the same classes to protect |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
615 |
objects referenced from local variables as we do to project objects |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
616 |
referenced from data members. In an ideal world, the LManagedXxx |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
617 |
classes would be used in both scenarios (just like auto_ptr is |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
618 |
elsewhere); LCleanedupXxx would not be necessary. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
619 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
620 |
In fact, the LManagedXxx classes can be used to protect locals if you |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
621 |
wish: |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
622 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
623 |
@code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
624 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
625 |
LManagedPtr<CQuery> query(CQuery::NewL()); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
626 |
query->BuildQueryL(); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
627 |
query->ExecuteQueryL(); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
628 |
} // query deleted on normal scope exit or leave |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
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diff
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|
629 |
@endcode |
e4d67989cc36
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
630 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
631 |
This works fine in isolation. Unfortunately, cleanup order becomes |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
632 |
counter-intuitive if calls to functions that use LManagedXxx are mixed |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
633 |
with functions that use the cleanup stack with the same call stack. |
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Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
634 |
|
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
635 |
Here is an example. Call order is top-to-bottom. |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
636 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
637 |
@code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
638 |
void CFramework::StartMainLoopL() |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
639 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
640 |
CFramework* fw = CFramework::NewL(); |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
641 |
CleanupStack::PushL(fw); // #1 |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
642 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
643 |
CleanupStack::PushL(event); // #2 |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
644 |
fw->DispatchL(*event); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
645 |
CleanupStack::PopAndDestroy(event); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
646 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
647 |
CleanupStack::PopAndDestroy(fw); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
648 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
649 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
650 |
void CFramework::DispatchL(CEvent& aEvent) |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
651 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
652 |
this->UserCallback()(*this, aEvent); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
653 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
654 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
655 |
void MyUserCallback(CFramework& aFw, CEvent& aEvent) |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
656 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
657 |
LManagedPtr<CMyEventWrapper> myEvent(CMyEventWrapper::NewL(aEvent)); // #3 |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
658 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
659 |
aFw->LookupL(arg); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
660 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
661 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
662 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
663 |
void CFramework::LookupL(const TDesC& aArg) |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
664 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
665 |
// |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
666 |
CleanupStack::PushL(tmp); // #4 |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
667 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
668 |
User::LeaveIfError(status); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
669 |
// ... |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
670 |
CleanupStack::PopAndDestroy(tmp); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
671 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
672 |
@endcode |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
673 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
674 |
If the User::LeaveIfError() expression triggers an unhandled leave in |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
675 |
the above, the cleanup order would be: #4, #2, #1, #3. If out-of-order |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
676 |
execution of CMyEventWrapper's destructor can never cause problems, |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
677 |
then there is not an issue. However, if CMyEventWrapper's destructor |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
678 |
were to e.g. decrement a reference count on the (deleted) CEvent, this |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
679 |
would lead to a panic. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
680 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
681 |
The cleanup order is this way because of the way User::Leave() |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
682 |
processing works. It locates all pushed CleanupStack items up to the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
683 |
point of the handling TRAP and executes their cleanups. Then, finally, |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
684 |
it uses C++ throw to unwind the C stack to the point of the TRAP. It |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
685 |
is during C stack unwinding that the destructors for objects stored in |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
686 |
locals get run, and LManagedPtr (like auto_ptr) relies exclusively on |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
687 |
destructor execution to trigger cleanup. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
688 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
689 |
On the other hand, if MyUserCallback uses LCleanedupPtr instead of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
690 |
LManagedPtr, then its associated cleanup is triggered as part of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
691 |
CleanupStack processing and in the more intuitive sequence: #4, #3, |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
692 |
#2, #1. Note that although it also has a destructor, LCleanedupPtr's |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
693 |
cleanup action is only ever run once; execution of the cleanup action |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
694 |
via the destructor is disabled if the cleanup action has already been |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
695 |
run as part of CleanupStack processing. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
696 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
697 |
This is why the LCleanedupXxx classes exist. Because the situations |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
698 |
where use of LManagedXxx for locals (or more precisely the resulting |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
699 |
out-of-order cleanup) may cause problems are subtle to characterise, |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
700 |
we guide conservatively that LCleanedupXxx should always be used to |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
701 |
protect locals. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
702 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
703 |
\b Cleanedup.2 <em>If you are forced to call an LC method, how should you deal with it?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
704 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
705 |
APIs should always offer pure L variants (after all, what if you want |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
706 |
to store the result directly in a data member?) and this scenario |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
707 |
should be rare. If it does arise, however, you have a few options. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
708 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
709 |
Note that code checking tools like Leavescan can flag dangerous |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
710 |
combinations of LC method calls and the LCleanedupXxx classes within |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
711 |
the same function. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
712 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
713 |
Option 1 (preferred): |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
714 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
715 |
@code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
716 |
// Define your own popping L wrapper function |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
717 |
CFoo* NewCFooL(TFooArg aArg) |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
718 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
719 |
CFoo* foo = CFoo::NewLC(aArg); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
720 |
CleanupStack::Pop(); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
721 |
return foo; |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
722 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
723 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
724 |
// Then call that instead... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
725 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
726 |
LCleanedupPtr<CFoo> foo(NewCFooL(arg)); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
727 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
728 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
729 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
730 |
@endcode |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
731 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
732 |
Option 2: |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
733 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
734 |
@code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
735 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
736 |
LCleanedupPtr<CFoo> foo; // pushes our "smart" cleanup item on the stack |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
737 |
foo = CFoo::NewLC(arg); // leaves a classic cleanup item on the stack |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
738 |
CleanupStack::Pop(); // gets rid of the classic cleanup item right away |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
739 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
740 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
741 |
@endcode |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
742 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
743 |
Never do this: |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
744 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
745 |
@code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
746 |
{ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
747 |
LCleanedupPtr<CFoo> foo(CFoo::NewLC(arg)); |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
748 |
// Execution order of the above statement leaves our "smart" cleanup item |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
749 |
// on top of the cleanup stack and we can't get at the LC-introduced cleanup |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
750 |
// item beneath it to pop it |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
751 |
// ... |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
752 |
} |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
753 |
@endcode |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
754 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
755 |
\b Cleanedup.3 <em>How do you create your own custom cleanup operation a la TCleanupItem?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
756 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
757 |
The direct analog to pushing a TCleanupItem is ::LCleanedupGuard; see |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
758 |
its documentation for an example of how to register a function pointer |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
759 |
with argument data to get invoked on cleanup. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
760 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
761 |
See also #DEFINE_CLEANUP_FUNCTION and #DEFINE_CLEANUP_STRATEGY for |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
762 |
creating custom cleanup operations for use with particular classes. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
763 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
764 |
\b Managed.1 <em>For LManagedXxx, is destruction order based on order of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
765 |
declaration?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
766 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
767 |
Yes, fields are destroyed (and so the cleanup operation run in the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
768 |
case of LManagedXxx fields) in reverse order of declaration as |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
769 |
specified by the C++ standard. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
770 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
771 |
\b Managed.2 <em>Does tying cleanup order to data member order make it |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
772 |
harder to maintain binary compatibility? If I want to change |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
773 |
construction/destruction order I'm forced to change my header and |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
774 |
reorder my data members?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
775 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
776 |
Reordering private fields would not constitute a BC break because the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
777 |
class would not change size. In situations where your data members are |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
778 |
anything other than private, including being directly revealed by |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
779 |
inline methods, you always have to take great care, and will continue |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
780 |
to have to do so. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
781 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
782 |
As a general guideline, if you are concerned about BC never make data |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
783 |
members (LManagedXxx or otherwise) anything other than private. Always |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
784 |
expose them via non-inline accessor methods, both to clients and to |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
785 |
derived classes. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
786 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
787 |
\b Managed.3 <em>What happens when you mix managed and unmanaged |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
788 |
pointers in the same class? Should it be avoided?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
789 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
790 |
Your manually-written destructor code for a class will get run before |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
791 |
the managed fields of that class get cleaned up, as per the C++ |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
792 |
standard. As long as this is understood, there is no particular reason |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
793 |
to avoid mixing if it makes sense to your problem. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
794 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
795 |
Note, though, that if you are using single-phase construction and the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
796 |
constructor leaves, your destructor will not be run; only the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
797 |
instance's fully-constructed data members at the point of the leave |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
798 |
will be destroyed. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
799 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
800 |
In some cases using ::LManagedGuard may be preferable to adding code |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
801 |
to the destructor. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
802 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
803 |
\b SinglePhase.1 <em>Do you have to use single-phase construction with |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
804 |
these APIs?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
805 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
806 |
No. The primary benefit of using the LManagedXxx classes to look after |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
807 |
your data members is to avoid having to write and maintain destructor |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
808 |
code manually. It's fine to initialize your LManagedXxx fields by |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
809 |
assignment in ConstructL if you're using two-phase construction; the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
810 |
automatic cleanup benefit is the same. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
811 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
812 |
The LManagedXxx classes also happen to enable the use of single-phase |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
813 |
construction, but you need to consider carefully when and whether to |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
814 |
use it. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
815 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
816 |
\b SinglePhase.2 <em>The single-phase constructor solution with |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
817 |
CONSTRUCTORS_MAY_LEAVE is a bit of a mess, and not really helping to |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
818 |
simplify code?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
819 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
820 |
Yes. Unfortunately the need for #CONSTRUCTORS_MAY_LEAVE is forced by |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
821 |
Symbian's legacy handling new(ELeave). |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
822 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
823 |
\b SinglePhase.3 <em>Why and where would I ever use single-phase |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
824 |
construction?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
825 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
826 |
For classes not intended for derivation there is no issue, and it's |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
827 |
quite reasonable to use the more concise single-phase construction |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
828 |
idiom, typically still hidden behind a NewL. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
829 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
830 |
For abstract framework classes intended for derivation outside the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
831 |
defining module, the single-phase/two-phase distinction would need to |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
832 |
be clearly documented and so would force awareness of at least some of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
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|
833 |
the issues described here, even in scenarios where the deriving client |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
834 |
was not intending to use the new idioms. This is probably not |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
835 |
desirable. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
836 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
837 |
\b OrLeave.1 <em>Can you still use User::LeaveIfError() now that |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
838 |
OR_LEAVE is available? What if you want to locally-handle some error |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
839 |
cases but not others.</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
840 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
841 |
Yes, #OR_LEAVE is simply a convenience macro that allows you to |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
842 |
deemphasize auxiliary error checking code in most cases. You can still |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
843 |
bind an error code manually, check it, and then use |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
844 |
User::LeaveIfError(). |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
845 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
846 |
\b Strings.1 <em>Why derive LString from RBuf and then hide parts of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
847 |
the inherited RBuf/TDes API?</em> |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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848 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
849 |
We want maximum interoperability with existing code and APIs for |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
850 |
LString in order to maximise its usefulness. In order to achieve that |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
851 |
we derive from RBuf, allowing an LString instance to be passed |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
852 |
directly as a const TDesC, a TDes, or an RBuf. |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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853 |
|
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
854 |
The trade-off is that parts of the RBuf API don't fit well with |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
855 |
LString. To be conservative, we have hidden anything inherited from |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
856 |
RBuf or TDes that we have found to be confusing to direct users of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
857 |
LString (you can add things later to a new API, but you can less |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
858 |
easily take them away). The most obvious of these is the mass hiding |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
859 |
of of the non-leaving descriptor methods for which LString provides |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
860 |
leaving, auto-growing alternatives: e.g. for direct users of LString, |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
861 |
we hide Append() so that it can't be used accidentally instead of |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
862 |
AppendL(). |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
863 |
|
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
864 |
Note that when passed as a TDes or RBuf, and viewed through a variable |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
865 |
of that type, this localised hiding within LString does not have any |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
866 |
effect; existing code handed an LString will be able to manipulate it |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
867 |
as it expects to. |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
868 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
869 |
\b Strings.2 <em>Does LString support TDesC/TDes features like char* |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
870 |
interop?</em> |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
871 |
|
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
872 |
LString is an RBuf, a TDes, and a TDesC, and retains all the |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
873 |
functionality of those classes. |
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parents:
diff
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874 |
|
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
875 |
LString8 retains the same level of char* interop as existing |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
876 |
descriptor classes through support for initialization from TUint8* |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
877 |
zero-terminated C strings, the ZeroTerminateL() utility method, Ptr(), |
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
878 |
and so on. |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
879 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
880 |
\b Strings.3 <em>Does LString auto-compress as well as auto-grow?</em> |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
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|
881 |
|
e4d67989cc36
Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
changeset
|
882 |
They don't. It can be hard to avoid pathologies when second-guessing |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
883 |
when client code if finished with buffer capacity. |
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
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|
884 |
|
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Revision: 201002
Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
parents:
diff
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|
885 |
*/ |