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1 /**************************************************************************** |
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2 ** |
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3 ** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies). |
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4 ** All rights reserved. |
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5 ** Contact: Nokia Corporation (qt-info@nokia.com) |
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6 ** |
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7 ** This file is part of the documentation of the Qt Toolkit. |
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8 ** |
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9 ** $QT_BEGIN_LICENSE:LGPL$ |
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10 ** No Commercial Usage |
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11 ** This file contains pre-release code and may not be distributed. |
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12 ** You may use this file in accordance with the terms and conditions |
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13 ** contained in the Technology Preview License Agreement accompanying |
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14 ** this package. |
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15 ** |
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16 ** GNU Lesser General Public License Usage |
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17 ** Alternatively, this file may be used under the terms of the GNU Lesser |
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18 ** General Public License version 2.1 as published by the Free Software |
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19 ** Foundation and appearing in the file LICENSE.LGPL included in the |
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20 ** packaging of this file. Please review the following information to |
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21 ** ensure the GNU Lesser General Public License version 2.1 requirements |
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22 ** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. |
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23 ** |
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24 ** In addition, as a special exception, Nokia gives you certain additional |
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25 ** rights. These rights are described in the Nokia Qt LGPL Exception |
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26 ** version 1.1, included in the file LGPL_EXCEPTION.txt in this package. |
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28 ** If you have questions regarding the use of this file, please contact |
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29 ** Nokia at qt-info@nokia.com. |
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30 ** |
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31 ** |
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32 ** |
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33 ** |
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34 ** |
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35 ** |
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36 ** |
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37 ** |
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38 ** $QT_END_LICENSE$ |
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39 ** |
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40 ****************************************************************************/ |
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41 |
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42 /*! |
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43 \page exceptionsafety.html |
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44 \title Exception Safety |
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45 \ingroup best-practices |
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46 \brief A guide to exception safety in Qt. |
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47 |
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48 \bold {Preliminary warning}: Exception safety is not feature complete! |
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49 Common cases should work, but classes might still leak or even crash. |
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50 |
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51 Qt itself will not throw exceptions. Instead, error codes are used. |
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52 In addition, some classes have user visible error messages, for example |
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53 \l QIODevice::errorString() or \l QSqlQuery::lastError(). |
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54 This has historical and practical reasons - turning on exceptions |
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55 can increase the library size by over 20%. |
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56 |
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57 The following sections describe Qt's behavior if exception support is |
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58 enabled at compile time. |
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59 |
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60 \tableofcontents |
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61 |
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62 \section1 Exception safe modules |
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63 |
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64 \section2 Containers |
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65 |
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66 Qt's \l{container classes} are generally exception neutral. They pass any |
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67 exception that happens within their contained type \c T to the user |
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68 while keeping their internal state valid. |
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69 |
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70 Example: |
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71 |
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72 \code |
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73 QList<QString> list; |
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74 ... |
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75 try { |
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76 list.append("hello"); |
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77 } catch (...) { |
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78 } |
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79 // list is safe to use - the exception did not affect it. |
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80 \endcode |
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81 |
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82 Exceptions to that rule are containers for types that can throw during assignment |
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83 or copy constructions. For those types, functions that modify the container as well as |
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84 returning a value, are unsafe to use: |
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85 |
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86 \code |
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87 MyType s = list.takeAt(2); |
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88 \endcode |
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89 |
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90 If an exception occurs during the assignment of \c s, the value at index 2 is already |
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91 removed from the container, but hasn't been assigned to \c s yet. It is lost |
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92 without chance of recovery. |
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93 |
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94 The correct way to write it: |
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95 |
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96 \code |
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97 MyType s = list.at(2); |
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98 list.removeAt(2); |
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99 \endcode |
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100 |
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101 If the assignment throws, the container still contains the value, no data loss occured. |
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102 |
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103 Note that implicitly shared Qt classes will not throw in their assignment |
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104 operators or copy constructors, so the limitation above does not apply. |
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105 |
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106 \section1 Out of Memory Handling |
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107 |
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108 Most desktop operating systems overcommit memory. This means that \c malloc() |
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109 or \c{operator new} return a valid pointer, even though there is not enough |
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110 memory available at allocation time. On such systems, no exception of type |
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111 \c std::bad_alloc is thrown. |
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112 |
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113 On all other operating systems, Qt will throw an exception of type std::bad_alloc |
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114 if any allocation fails. Allocations can fail if the system runs out of memory or |
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115 doesn't have enough continuous memory to allocate the requested size. |
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116 |
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117 Exceptions to that rule are documented. As an example, \l QImage::create() |
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118 returns false if not enough memory exists instead of throwing an exception. |
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119 |
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120 \section1 Recovering from exceptions |
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121 |
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122 Currently, the only supported use case for recovering from exceptions thrown |
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123 within Qt (for example due to out of memory) is to exit the event loop and do |
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124 some cleanup before exiting the application. |
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125 |
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126 Typical use case: |
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127 |
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128 \code |
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129 QApplication app(argc, argv); |
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130 ... |
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131 try { |
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132 app.exec(); |
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133 } catch (const std::bad_alloc &) { |
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134 // clean up here, e.g. save the session |
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135 // and close all config files. |
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136 |
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137 return 0; // exit the application |
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138 } |
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139 \endcode |
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140 |
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141 After an exception is thrown, the connection to the windowing server |
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142 might already be closed. It is not safe to call a GUI related function |
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143 after catching an exception. |
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144 |
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145 \section1 Platform-Specific Exception Handling |
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146 |
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147 \section2 The Symbian platform |
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148 |
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149 The Symbian platform implements its own exception system that differs from the standard |
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150 C++ mechanism. When using Qt for the Symbian platform, and especially when writing code to |
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151 access Symbian functionality directly, it may be necessary to know about the underlying |
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152 implementation and how it interacts with Qt. |
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153 |
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154 The \l{Exception Safety with Symbian} document shows how to use the facilities provided |
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155 by Qt to use exceptions as safely as possible. |
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156 */ |