docs/doxygen/overviews/refcount.h

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