docs/doxygen/overviews/runtimeclass.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        runtimeclass.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows licence
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /**
  10
  11 @page overview_rtti Runtime Type Information (RTTI)
  12
  13 @li @ref overview_rtti_intro
  14 @li @ref overview_rtti_classinfo
  15 @li @ref overview_rtti_example
  16
  17 @see
  18
  19 @li wxObject
  20 @li wxClassInfo
  21
  22
  23 <hr>
  24
  25
  26 @section overview_rtti_intro Introduction
  27
  28 One of the failings of C++ used to be that no runtime information was provided
  29 about a class and its position in the inheritance hierarchy. Another, which
  30 still persists, is that instances of a class cannot be created just by knowing
  31 the name of a class, which makes facilities such as persistent storage hard to
  32 implement.
  33
  34 Most C++ GUI frameworks overcome these limitations by means of a set of macros
  35 and functions and wxWidgets is no exception. As it originated before the
  36 addition of RTTI to the C++ standard and as support for it is still missing
  37 from some (albeit old) compilers, wxWidgets doesn't (yet) use it, but provides
  38 its own macro-based RTTI system.
  39
  40 In the future, the standard C++ RTTI will be used though and you're encouraged
  41 to use whenever possible the wxDynamicCast macro which, for the implementations
  42 that support it, is defined just as dynamic_cast and uses wxWidgets RTTI for
  43 all the others. This macro is limited to wxWidgets classes only and only works
  44 with pointers (unlike the real dynamic_cast which also accepts references).
  45
  46 Each class that you wish to be known to the type system should have a macro
  47 such as DECLARE_DYNAMIC_CLASS just inside the class declaration. The macro
  48 IMPLEMENT_DYNAMIC_CLASS should be in the implementation file. Note that these
  49 are entirely optional; use them if you wish to check object types, or create
  50 instances of classes using the class name. However, it is good to get into the
  51 habit of adding these macros for all classes.
  52
  53 Variations on these macros are used for multiple inheritance, and abstract
  54 classes that cannot be instantiated dynamically or otherwise.
  55
  56 DECLARE_DYNAMIC_CLASS inserts a static wxClassInfo declaration into the class,
  57 initialized by IMPLEMENT_DYNAMIC_CLASS. When initialized, the wxClassInfo
  58 object inserts itself into a linked list (accessed through wxClassInfo::first
  59 and wxClassInfo::next pointers). The linked list is fully created by the time
  60 all global initialisation is done.
  61
  62 IMPLEMENT_DYNAMIC_CLASS is a macro that not only initialises the static
  63 wxClassInfo member, but defines a global function capable of creating a dynamic
  64 object of the class in question. A pointer to this function is stored in
  65 wxClassInfo, and is used when an object should be created dynamically.
  66
  67 wxObject::IsKindOf uses the linked list of wxClassInfo. It takes a wxClassInfo
  68 argument, so use CLASSINFO(className) to return an appropriate wxClassInfo
  69 pointer to use in this function.
  70
  71 The function wxCreateDynamicObject can be used to construct a new object of a
  72 given type, by supplying a string name. If you have a pointer to the
  73 wxClassInfo object instead, then you can simply call wxClassInfo::CreateObject.
  74
  75
  76 @section overview_rtti_classinfo wxClassInfo
  77
  78 This class stores meta-information about classes. An application may use macros
  79 such as DECLARE_DYNAMIC_CLASS and IMPLEMENT_DYNAMIC_CLASS to record runtime
  80 information about a class, including:
  81
  82 @li Its position in the inheritance hierarchy.
  83 @li The base class name(s) (up to two base classes are permitted).
  84 @li A string representation of the class name.
  85 @li A function that can be called to construct an instance of this class.
  86
  87 The DECLARE_... macros declare a static wxClassInfo variable in a class, which
  88 is initialized by macros of the form IMPLEMENT_... in the implementation C++
  89 file. Classes whose instances may be constructed dynamically are given a global
  90 constructor function which returns a new object.
  91
  92 You can get the wxClassInfo for a class by using the CLASSINFO macro, e.g.
  93 CLASSINFO(wxFrame). You can get the wxClassInfo for an object using
  94 wxObject::GetClassInfo.
  95
  96 @see
  97
  98 @li wxObject
  99 @li wxCreateDynamicObject
 100
 101
 102 @section overview_rtti_example Example
 103
 104 In a header file frame.h:
 105
 106 @code
 107 class wxFrame : public wxWindow
 108 {
 109     DECLARE_DYNAMIC_CLASS(wxFrame)
 110
 111 private:
 112     wxString m_title;
 113
 114 public:
 115 ...
 116 };
 117 @endcode
 118
 119 In a C++ file frame.cpp:
 120
 121 @code
 122 IMPLEMENT_DYNAMIC_CLASS(wxFrame, wxWindow)
 123
 124 wxFrame::wxFrame()
 125 {
 126 ...
 127 }
 128 @endcode
 129
 130 */
 131