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40 ****************************************************************************/ |
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41 |
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42 /* TODO: Move some of the documentation from QSharedDataPointer into this |
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43 document. */ |
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44 |
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45 /*! |
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46 \group shared |
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47 \title Implicitly Shared Classes |
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48 */ |
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49 |
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50 /*! |
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51 \page implicit-sharing.html |
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52 \title Implicit Sharing |
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53 \ingroup frameworks-technologies |
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54 |
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55 \brief Reference counting for fast copying. |
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56 |
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57 \keyword implicit data sharing |
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58 \keyword implicit sharing |
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59 \keyword implicitly shared |
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60 \keyword reference counting |
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61 \keyword shared implicitly |
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62 \keyword shared classes |
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63 |
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64 Many C++ classes in Qt use implicit data sharing to maximize |
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65 resource usage and minimize copying. Implicitly shared classes are |
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66 both safe and efficient when passed as arguments, because only a |
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67 pointer to the data is passed around, and the data is copied only |
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68 if and when a function writes to it, i.e., \e {copy-on-write}. |
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69 |
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70 \tableofcontents |
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71 |
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72 \section1 Overview |
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73 |
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74 A shared class consists of a pointer to a shared data block that |
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75 contains a reference count and the data. |
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76 |
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77 When a shared object is created, it sets the reference count to 1. The |
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78 reference count is incremented whenever a new object references the |
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79 shared data, and decremented when the object dereferences the shared |
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80 data. The shared data is deleted when the reference count becomes |
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81 zero. |
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82 |
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83 \keyword deep copy |
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84 \keyword shallow copy |
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85 |
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86 When dealing with shared objects, there are two ways of copying an |
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87 object. We usually speak about \e deep and \e shallow copies. A deep |
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88 copy implies duplicating an object. A shallow copy is a reference |
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89 copy, i.e. just a pointer to a shared data block. Making a deep copy |
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90 can be expensive in terms of memory and CPU. Making a shallow copy is |
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91 very fast, because it only involves setting a pointer and incrementing |
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92 the reference count. |
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93 |
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94 Object assignment (with operator=()) for implicitly shared objects is |
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95 implemented using shallow copies. |
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96 |
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97 The benefit of sharing is that a program does not need to duplicate |
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98 data unnecessarily, which results in lower memory use and less copying |
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99 of data. Objects can easily be assigned, sent as function arguments, |
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100 and returned from functions. |
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101 |
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102 Implicit sharing takes place behind the scenes; the programmer |
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103 does not need to worry about it. Even in multithreaded |
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104 applications, implicit sharing takes place, as explained in |
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105 \l{Thread-Support in Qt Modules#Threads and Implicitly Shared Classes} |
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106 {Threads and Implicitly Shared Classes}. |
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107 |
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108 When implementing your own implicitly shared classes, use the |
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109 QSharedData and QSharedDataPointer classes. |
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110 |
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111 \section1 Implicit Sharing in Detail |
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112 |
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113 Implicit sharing automatically detaches the object from a shared |
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114 block if the object is about to change and the reference count is |
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115 greater than one. (This is often called \e {copy-on-write} or |
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116 \e {value semantics}.) |
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117 |
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118 An implicitly shared class has total control of its internal data. In |
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119 any member functions that modify its data, it automatically detaches |
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120 before modifying the data. |
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121 |
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122 The QPen class, which uses implicit sharing, detaches from the shared |
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123 data in all member functions that change the internal data. |
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124 |
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125 Code fragment: |
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126 \snippet doc/src/snippets/code/doc_src_groups.qdoc 0 |
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127 |
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128 |
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129 \section1 List of Classes |
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130 |
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131 The classes listed below automatically detach from common data if |
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132 an object is about to be changed. The programmer will not even |
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133 notice that the objects are shared. Thus you should treat |
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134 separate instances of them as separate objects. They will always |
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135 behave as separate objects but with the added benefit of sharing |
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136 data whenever possible. For this reason, you can pass instances |
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137 of these classes as arguments to functions by value without |
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138 concern for the copying overhead. |
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139 |
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140 Example: |
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141 \snippet doc/src/snippets/code/doc_src_groups.qdoc 1 |
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142 |
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143 In this example, \c p1 and \c p2 share data until QPainter::begin() |
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144 is called for \c p2, because painting a pixmap will modify it. |
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145 |
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146 \warning Do not copy an implicitly shared container (QMap, |
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147 QVector, etc.) while you are iterating over it using an non-const |
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148 \l{STL-style iterator}. |
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149 |
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150 \keyword implicitly shared classes |
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151 \annotatedlist shared |
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152 */ |