94 lines
3.5 KiB
TeX
94 lines
3.5 KiB
TeX
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\section{Run time class information overview}\label{runtimeclassoverview}
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Classes: \helpref{wxObject}{wxobject}, \helpref{wxClassInfo}{wxclassinfo}.
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One of the failings of C++ is that no run-time information is provided
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about a class and its position in the inheritance hierarchy.
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Another is that instances of a class cannot be created just by knowing the name of a class,
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which makes facilities such as persistent storage hard to implement.
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Most C++ GUI frameworks overcome these limitations by means of a set of
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macros and functions and wxWindows (from version 1.62) is no exception.
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Each class that you wish to be known the type system should have
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a macro such as DECLARE\_DYNAMIC\_CLASS just inside the class declaration.
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The macro IMPLEMENT\_DYNAMIC\_CLASS should be in the implementation file.
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Variations on these \helpref{macros}{macros} are used for multiple inheritance, and abstract
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classes that cannot be instantiated dynamically or otherwise.
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DECLARE\_DYNAMIC\_CLASS inserts a static wxClassInfo declaration into the
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class, initialized by IMPLEMENT\_DYNAMIC\_CLASS. When initialized, the
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wxClassInfo object inserts itself into a linked list (accessed through
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wxClassInfo::first and wxClassInfo::next pointers). The linked list
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is fully created by the time all global initialisation is done.
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IMPLEMENT\_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
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dynamic object of the class in question. A pointer to this function is
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stored in wxClassInfo, and is used when an object should be created
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dynamically.
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wxObject::IsKindOf uses the linked list of wxClassInfo. It takes
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a wxClassInfo argument, so use CLASSINFO(className) to return an
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appropriate wxClassInfo pointer to use in this function.
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The function \helpref{wxCreateDynamicObject}{wxcreatedynamicobject} can be used
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to construct a new object of a given type, by supplying a string name.
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If you have a pointer to the wxClassInfo object instead, then you
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can simply call wxClassInfo::CreateObject.
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\subsection{wxClassInfo}\label{wxclassinfooverview}
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\overview{Run time class information overview}{runtimeclassoverview}
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Class: \helpref{wxClassInfo}{wxclassinfo}
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This class stores meta-information about classes. An application
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may use macros such as DECLARE\_DYNAMIC\_CLASS and IMPLEMENT\_DYNAMIC\_CLASS
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to record run-time information about a class, including:
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\begin{itemize}\itemsep=0pt
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\item its position in the inheritance hierarchy;
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\item the base class name(s) (up to two base classes are permitted);
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\item a string representation of the class name;
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\item a function that can be called to construct an instance of this class.
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\end{itemize}
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The DECLARE\_... macros declare a static wxClassInfo variable in a class, which is initialized
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by macros of the form IMPLEMENT\_... in the implementation C++ file. Classes whose instances may be
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constructed dynamically are given a global 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. CLASSINFO(wxFrame).
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You can get the wxClassInfo for an object using wxObject::GetClassInfo.
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See also \helpref{wxObject}{wxobject} and \helpref{wxCreateDynamicObject}{wxcreatedynamicobject}.
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\subsection{Example}
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In a header file wx\_frame.h:
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\begin{verbatim}
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class wxFrame: public wxWindow
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{
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DECLARE_DYNAMIC_CLASS(wxFrame)
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private:
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char *frameTitle;
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public:
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...
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};
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\end{verbatim}
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In a C++ file wx\_frame.cc:
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\begin{verbatim}
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IMPLEMENT_DYNAMIC_CLASS(wxFrame, wxWindow)
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wxFrame::wxFrame(void)
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{
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...
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}
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\end{verbatim}
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