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[wxWidgets.git] / docs / doxygen / overviews / runtimeclass.h

/////////////////////////////////////////////////////////////////////////////
// Name:        runtimeclass
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
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/*!

 @page runtimeclass_overview Runtime class information (aka RTTI) overview

 Classes: #wxObject, #wxClassInfo.
 One of the failings of C++ used to be that no run-time information was provided
 about a class and its position in the inheritance hierarchy.
 Another, which still persists, is that instances of a class cannot be created
 just by knowing the name of a class, which makes facilities such as persistent
 storage hard to implement.
 Most C++ GUI frameworks overcome these limitations by means of a set of
 macros and functions and wxWidgets is no exception. As it originated before the
 addition of RTTI to the C++ standard and as support for it is still missing from
 some (albeit old) compilers, wxWidgets doesn't (yet) use it, but provides its
 own macro-based RTTI system.
 In the future, the standard C++ RTTI will be used though and you're encouraged
 to use whenever possible the #wxDynamicCast() macro which,
 for the implementations that support it, is defined just as dynamic_cast and
 uses wxWidgets RTTI for all the others. This macro is limited to wxWidgets
 classes only and only works with pointers (unlike the real dynamic_cast which
 also accepts references).
 Each class that you wish to be known to the type system should have
 a macro such as DECLARE_DYNAMIC_CLASS just inside the class declaration.
 The macro IMPLEMENT_DYNAMIC_CLASS should be in the implementation file.
 Note that these are entirely optional; use them if you wish to check object
 types, or create instances of classes using the class name. However,
 it is good to get into the habit of adding these macros for all classes.
 Variations on these #macros are used for multiple inheritance, and abstract
 classes that cannot be instantiated dynamically or otherwise.
 DECLARE_DYNAMIC_CLASS inserts a static wxClassInfo declaration into the
 class, initialized by IMPLEMENT_DYNAMIC_CLASS. When initialized, the
 wxClassInfo object inserts itself into a linked list (accessed through
 wxClassInfo::first and wxClassInfo::next pointers). The linked list
 is fully created by the time all global initialisation is done.
 IMPLEMENT_DYNAMIC_CLASS is a macro that not only initialises the static
 wxClassInfo member, but defines a global function capable of creating a
 dynamic object of the class in question. A pointer to this function is
 stored in wxClassInfo, and is used when an object should be created
 dynamically.
 wxObject::IsKindOf uses the linked list of
 wxClassInfo. It takes a wxClassInfo argument, so use CLASSINFO(className)
 to return an appropriate wxClassInfo pointer to use in this function.
 The function #wxCreateDynamicObject can be used
 to construct a new object of a given type, by supplying a string name.
 If you have a pointer to the wxClassInfo object instead, then you
 can simply call wxClassInfo::CreateObject.
 #wxClassInfo
 #Example


 @section wxclassinfooverview wxClassInfo

 #Runtime class information (aka RTTI) overview
 Class: #wxClassInfo
 This class stores meta-information about classes. An application
 may use macros such as DECLARE_DYNAMIC_CLASS and IMPLEMENT_DYNAMIC_CLASS
 to record run-time information about a class, including:


  its position in the inheritance hierarchy;
  the base class name(s) (up to two base classes are permitted);
  a string representation of the class name;
  a function that can be called to construct an instance of this class.


 The DECLARE_... macros declare a static wxClassInfo variable in a class, which is initialized
 by macros of the form IMPLEMENT_... in the implementation C++ file. Classes whose instances may be
 constructed dynamically are given a global constructor function which returns a new object.
 You can get the wxClassInfo for a class by using the CLASSINFO macro, e.g. CLASSINFO(wxFrame).
 You can get the wxClassInfo for an object using wxObject::GetClassInfo.
 See also #wxObject and #wxCreateDynamicObject.

 @section runtimeclassinformationexample Example

 In a header file frame.h:

 @code
 class wxFrame : public wxWindow
 {
 DECLARE_DYNAMIC_CLASS(wxFrame)

 private:
     wxString m_title;

 public:
     ...
 };
 @endcode

 In a C++ file frame.cpp:

 @code
 IMPLEMENT_DYNAMIC_CLASS(wxFrame, wxWindow)

 wxFrame::wxFrame()
 {
 ...
 }
 @endcode

 */