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