docs/doxygen/overviews/trefcount.h

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