124 lines
4.5 KiB
C
124 lines
4.5 KiB
C
/////////////////////////////////////////////////////////////////////////////
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// Name: refcount.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_refcount Reference Counting
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@tableofcontents
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Many wxWidgets objects use a technique known as <em>reference counting</em>,
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also known as <em>copy on write</em> (COW). This means that when an object is
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assigned to another, no copying really takes place. Only the reference count on
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the shared object data is incremented and both objects share the same data (a
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very fast operation).
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But as soon as one of the two (or more) objects is modified, the data has to be
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copied because the changes to one of the objects shouldn't be seen in the
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others. As data copying only happens when the object is written to, this is
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known as COW.
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What is important to understand is that all this happens absolutely
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transparently to the class users and that whether an object is shared or not is
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not seen from the outside of the class - in any case, the result of any
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operation on it is the same.
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@section overview_refcount_equality Object Comparison
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The == and != operators of @ref overview_refcount_list "the reference counted classes"
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always do a <em>deep comparison</em>. This means that the equality operator
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will return @true if two objects are identical and not only if they share the
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same data.
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Note that wxWidgets follows the <em>STL philosophy</em>: when a comparison
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operator cannot be implemented efficiently (like for e.g. wxImage's ==
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operator which would need to compare the entire image's data, pixel-by-pixel),
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it's not implemented at all. That's why not all reference counted classes
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provide comparison operators.
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Also note that if you only need to do a @c shallow comparison between two
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wxObject derived classes, you should not use the == and != operators but
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rather the wxObject::IsSameAs() function.
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@section overview_refcount_destruct Object Destruction
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When a COW object destructor is called, it may not delete the data: if it's
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shared, the destructor will just decrement the shared data's reference count
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without destroying it. Only when the destructor of the last object owning the
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data is called, the data is really destroyed. Just like all other COW-things,
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this happens transparently to the class users so that you shouldn't care about
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it.
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@section overview_refcount_list List of Reference Counted Classes
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The following classes in wxWidgets have efficient (i.e. fast) assignment
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operators and copy constructors since they are reference-counted:
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@li wxAcceleratorTable
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@li wxAnimation
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@li wxBitmap
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@li wxBrush
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@li wxCursor
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@li wxFont
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@li wxGraphicsBrush
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@li wxGraphicsContext
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@li wxGraphicsFont
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@li wxGraphicsMatrix
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@li wxGraphicsPath
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@li wxGraphicsPen
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@li wxIcon
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@li wxImage
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@li wxMetafile
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@li wxPalette
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@li wxPen
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@li wxRegion
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@li wxVariant
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@li wxVariantData
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Note that the list above reports the objects which are reference counted in all
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ports of wxWidgets; some ports may use this technique also for other classes.
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All the objects implement a function @b IsOk() to test if they are referencing
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valid data; when the objects are in uninitialized state, you can only use the
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@b IsOk() getter; trying to call any other getter, e.g. wxBrush::GetStyle() on
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the ::wxNullBrush object, will result in an assert failure in debug builds.
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@section overview_refcount_object Making Your Own Reference Counted Class
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Reference counting can be implemented easily using wxObject or using the
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intermediate wxRefCounter class directly. Alternatively, you can also use the
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wxObjectDataPtr<T> template.
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First, derive a new class from wxRefCounter (or wxObjectRefData when using a
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wxObject derived class) and put the memory-consuming data in it.
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Then derive a new class from wxObject and implement there the public interface
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which will be seen by the user of your class. You'll probably want to add a
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function to your class which does the cast from wxObjectRefData to your
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class-specific shared data. For example:
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@code
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MyClassRefData* GetData() const
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{
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return static_cast<MyClassRefData*>(m_refData);
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}
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@endcode
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In fact, any time you need to read the data from your wxObject-derived class,
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you will need to call this function.
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@note Any time you need to actually modify the data placed inside your wxObject
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derived class, you must first call the wxObject::UnShare() function to ensure
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that the modifications won't affect other instances which are eventually
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sharing your object's data.
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*/
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