docs/doxygen/overviews/runtimeclass.h

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