114 lines
4.2 KiB
C
114 lines
4.2 KiB
C
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/////////////////////////////////////////////////////////////////////////////
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// Name: runtimeclass.h
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// Purpose: topic overview
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// Author: wxWidgets team
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// Licence: wxWindows licence
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/////////////////////////////////////////////////////////////////////////////
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/**
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@page overview_rtti Runtime Type Information (RTTI)
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@tableofcontents
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One of the failings of C++ used to be that no runtime information was provided
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about a class and its position in the inheritance hierarchy. Another, which
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still persists, is that instances of a class cannot be created just by knowing
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the name of a class, which makes facilities such as persistent storage hard to
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implement.
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Most C++ GUI frameworks overcome these limitations by means of a set of macros
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and functions and wxWidgets is no exception. As it originated before the
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addition of RTTI to the C++ standard and as support for it is still missing
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from some (albeit old) compilers, wxWidgets doesn't (yet) use it, but provides
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its own macro-based RTTI system.
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In the future, the standard C++ RTTI will be used though and you're encouraged
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to use whenever possible the wxDynamicCast macro which, for the implementations
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that support it, is defined just as dynamic_cast and uses wxWidgets RTTI for
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all the others. This macro is limited to wxWidgets classes only and only works
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with pointers (unlike the real dynamic_cast which also accepts references).
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Each class that you wish to be known to the type system should have a macro
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such as wxDECLARE_DYNAMIC_CLASS just inside the class declaration. The macro
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wxIMPLEMENT_DYNAMIC_CLASS should be in the implementation file. Note that these
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are entirely optional; use them if you wish to check object types, or create
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instances of classes using the class name. However, it is good to get into the
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habit of adding these macros for all classes.
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Variations on these macros are used for multiple inheritance, and abstract
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classes that cannot be instantiated dynamically or otherwise.
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wxDECLARE_DYNAMIC_CLASS inserts a static wxClassInfo declaration into the class,
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initialized by wxIMPLEMENT_DYNAMIC_CLASS. When initialized, the wxClassInfo
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object inserts itself into a linked list (accessed through wxClassInfo::first
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and wxClassInfo::next pointers). The linked list is fully created by the time
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all global initialisation is done.
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wxIMPLEMENT_DYNAMIC_CLASS is a macro that not only initialises the static
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wxClassInfo member, but defines a global function capable of creating a dynamic
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object of the class in question. A pointer to this function is stored in
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wxClassInfo, and is used when an object should be created dynamically.
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wxObject::IsKindOf uses the linked list of wxClassInfo. It takes a wxClassInfo
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argument, so use CLASSINFO(className) to return an appropriate wxClassInfo
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pointer to use in this function.
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The function wxCreateDynamicObject can be used to construct a new object of a
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given type, by supplying a string name. If you have a pointer to the
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wxClassInfo object instead, then you can simply call wxClassInfo::CreateObject.
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@see wxObject
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@section overview_rtti_classinfo wxClassInfo
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This class stores meta-information about classes. An application may use macros
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such as wxDECLARE_DYNAMIC_CLASS and wxIMPLEMENT_DYNAMIC_CLASS to record runtime
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information about a class, including:
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@li Its position in the inheritance hierarchy.
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@li The base class name(s) (up to two base classes are permitted).
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@li A string representation of the class name.
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@li A function that can be called to construct an instance of this class.
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The wxDECLARE_... macros declare a static wxClassInfo variable in a class, which
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is initialized by macros of the form wxIMPLEMENT_... in the implementation C++
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file. Classes whose instances may be constructed dynamically are given a global
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constructor function which returns a new object.
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You can get the wxClassInfo for a class by using the CLASSINFO macro, e.g.
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CLASSINFO(wxFrame). You can get the wxClassInfo for an object using
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wxObject::GetClassInfo.
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@section overview_rtti_example Example
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In a header file frame.h:
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@code
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class wxFrame : public wxWindow
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{
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wxDECLARE_DYNAMIC_CLASS(wxFrame);
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private:
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wxString m_title;
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public:
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...
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};
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@endcode
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In a C++ file frame.cpp:
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@code
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wxIMPLEMENT_DYNAMIC_CLASS(wxFrame, wxWindow);
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wxFrame::wxFrame()
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{
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...
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}
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@endcode
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*/
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