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 All the objects implement a function IsOk() to test if they are referencing valid
  94 data; when the objects are in uninitialized state, you can only use the IsOk() getter;
  95 trying to call any other getter, e.g. wxBrush::GetStyle() on the ::wxNullBrush object,
  96 will result in an assert failure in debug builds.
  97
  98
  99 @section overview_refcount_object Making Your Own Reference Counted Class
 100
 101 Reference counting can be implemented easily using wxObject and wxObjectRefData
 102 classes. Alternatively, you can also use the wxObjectDataPtr<T> template.
 103
 104 First, derive a new class from wxObjectRefData and put there the
 105 memory-consuming data.
 106
 107 Then derive a new class from wxObject and implement there the public interface
 108 which will be seen by the user of your class. You'll probably want to add a
 109 function to your class which does the cast from wxObjectRefData to your
 110 class-specific shared data. For example:
 111
 112 @code
 113 MyClassRefData* GetData() const
 114 {
 115     return wx_static_cast(MyClassRefData*, m_refData);
 116 }
 117 @endcode
 118
 119 In fact, any time you need to read the data from your wxObject-derived class,
 120 you will need to call this function.
 121
 122 @note Any time you need to actually modify the data placed inside your wxObject
 123 derived class, you must first call the wxObject::UnShare function to ensure
 124 that the modifications won't affect other instances which are eventually
 125 sharing your object's data.
 126
 127 */
 128