docs/doxygen/overviews/refcount.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        refcount.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows license
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /*!
  10
  11 @page overview_refcount Reference Counting
  12
  13 @li @ref overview_refcount_ignore
  14 @li @ref overview_refcount_equality
  15 @li @ref overview_refcount_destruct
  16 @li @ref overview_refcount_list
  17 @li @ref overview_refcount_object
  18
  19
  20 <hr>
  21
  22
  23 @section overview_refcount_ignore Why You Shouldn't Care About It
  24
  25 Many wxWidgets objects use a technique known as <em>reference counting</em>,
  26 also known as <em>copy on write</em> (COW). This means that when an object is
  27 assigned to another, no copying really takes place. Only the reference count on
  28 the shared object data is incremented and both objects share the same data (a
  29 very fast operation).
  30
  31 But as soon as one of the two (or more) objects is modified, the data has to be
  32 copied because the changes to one of the objects shouldn't be seen in the
  33 others. As data copying only happens when the object is written to, this is
  34 known as COW.
  35
  36 What is important to understand is that all this happens absolutely
  37 transparently to the class users and that whether an object is shared or not is
  38 not seen from the outside of the class - in any case, the result of any
  39 operation on it is the same.
  40
  41
  42 @section overview_refcount_equality Object Comparison
  43
  44 The == and != operators of the reference counted classes always do a @c deep
  45 comparison. This means that the equality operator will return @true if two
  46 objects are identical and not only if they share the same data.
  47
  48 Note that wxWidgets follows the <em>STL philosophy</em>: when a comparison
  49 operator can not be implemented efficiently (like for e.g. wxImage's ==
  50 operator which would need to compare the entire image's data, pixel-by-pixel),
  51 it's not implemented at all. That's why not all reference counted classes
  52 provide comparison operators.
  53
  54 Also note that if you only need to do a @c shallow comparison between two
  55 #wxObject derived classes, you should not use the == and != operators but
  56 rather the wxObject::IsSameAs function.
  57
  58
  59 @section overview_refcount_destruct Object Destruction
  60
  61 When a COW object destructor is called, it may not delete the data: if it's
  62 shared, the destructor will just decrement the shared data's reference count
  63 without destroying it. Only when the destructor of the last object owning the
  64 data is called, the data is really destroyed. Just like all other COW-things,
  65 this happens transparently to the class users so that you shouldn't care about
  66 it.
  67
  68
  69 @section overview_refcount_list List of Reference Counted Classes
  70
  71 The following classes in wxWidgets have efficient (i.e. fast) assignment
  72 operators and copy constructors since they are reference-counted:
  73
  74 @li #wxAcceleratorTable
  75 @li #wxAnimation
  76 @li #wxBitmap
  77 @li #wxBrush
  78 @li #wxCursor
  79 @li #wxFont
  80 @li #wxIcon
  81 @li #wxImage
  82 @li #wxMetafile
  83 @li #wxPalette
  84 @li #wxPen
  85 @li #wxRegion
  86 @li #wxString
  87 @li #wxVariant
  88 @li #wxVariantData
  89
  90 Note that the list above reports the objects which are reference counted in all
  91 ports of wxWidgets; some ports may use this technique also for other classes.
  92
  93
  94 @section overview_refcount_object Making Your Own Reference Counted Class
  95
  96 Reference counting can be implemented easily using #wxObject and
  97 #wxObjectRefData classes. Alternatively, you can also use the
  98 #wxObjectDataPtr<T> template.
  99
 100 First, derive a new class from #wxObjectRefData and put there the
 101 memory-consuming data.
 102
 103 Then derive a new class from #wxObject and implement there the public interface
 104 which will be seen by the user of your class. You'll probably want to add a
 105 function to your class which does the cast from #wxObjectRefData to your
 106 class-specific shared data. For example:
 107
 108 @code
 109 MyClassRefData* GetData() const
 110 {
 111     return wx_static_cast(MyClassRefData*, m_refData);
 112 }
 113 @endcode
 114
 115 In fact, any time you need to read the data from your wxObject-derived class,
 116 you will need to call this function.
 117
 118 @note Any time you need to actually modify the data placed inside your wxObject
 119 derived class, you must first call the wxObject::UnShare function to ensure
 120 that the modifications won't affect other instances which are eventually
 121 sharing your object's data.
 122
 123 */
 124