660b61d7e3
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65097 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
137 lines
4.8 KiB
C++
137 lines
4.8 KiB
C++
/////////////////////////////////////////////////////////////////////////////
|
|
// Name: refcount.h
|
|
// Purpose: topic overview
|
|
// Author: wxWidgets team
|
|
// RCS-ID: $Id$
|
|
// Licence: wxWindows licence
|
|
/////////////////////////////////////////////////////////////////////////////
|
|
|
|
/**
|
|
|
|
@page overview_refcount Reference Counting
|
|
|
|
@li @ref overview_refcount_ignore
|
|
@li @ref overview_refcount_equality
|
|
@li @ref overview_refcount_destruct
|
|
@li @ref overview_refcount_list
|
|
@li @ref overview_refcount_object
|
|
|
|
|
|
<hr>
|
|
|
|
|
|
@section overview_refcount_ignore Why You Shouldn't Care About It
|
|
|
|
Many wxWidgets objects use a technique known as <em>reference counting</em>,
|
|
also known as <em>copy on write</em> (COW). This means that when an object is
|
|
assigned to another, no copying really takes place. Only the reference count on
|
|
the shared object data is incremented and both objects share the same data (a
|
|
very fast operation).
|
|
|
|
But as soon as one of the two (or more) objects is modified, the data has to be
|
|
copied because the changes to one of the objects shouldn't be seen in the
|
|
others. As data copying only happens when the object is written to, this is
|
|
known as COW.
|
|
|
|
What is important to understand is that all this happens absolutely
|
|
transparently to the class users and that whether an object is shared or not is
|
|
not seen from the outside of the class - in any case, the result of any
|
|
operation on it is the same.
|
|
|
|
|
|
@section overview_refcount_equality Object Comparison
|
|
|
|
The == and != operators of @ref overview_refcount_list "the reference counted classes"
|
|
always do a <em>deep comparison</em>. This means that the equality operator
|
|
will return @true if two objects are identical and not only if they share the
|
|
same data.
|
|
|
|
Note that wxWidgets follows the <em>STL philosophy</em>: when a comparison
|
|
operator can not be implemented efficiently (like for e.g. wxImage's ==
|
|
operator which would need to compare the entire image's data, pixel-by-pixel),
|
|
it's not implemented at all. That's why not all reference counted classes
|
|
provide comparison operators.
|
|
|
|
Also note that if you only need to do a @c shallow comparison between two
|
|
wxObject derived classes, you should not use the == and != operators but
|
|
rather the wxObject::IsSameAs() function.
|
|
|
|
|
|
@section overview_refcount_destruct Object Destruction
|
|
|
|
When a COW object destructor is called, it may not delete the data: if it's
|
|
shared, the destructor will just decrement the shared data's reference count
|
|
without destroying it. Only when the destructor of the last object owning the
|
|
data is called, the data is really destroyed. Just like all other COW-things,
|
|
this happens transparently to the class users so that you shouldn't care about
|
|
it.
|
|
|
|
|
|
@section overview_refcount_list List of Reference Counted Classes
|
|
|
|
The following classes in wxWidgets have efficient (i.e. fast) assignment
|
|
operators and copy constructors since they are reference-counted:
|
|
|
|
@li wxAcceleratorTable
|
|
@li wxAnimation
|
|
@li wxBitmap
|
|
@li wxBrush
|
|
@li wxCursor
|
|
@li wxFont
|
|
@li wxGraphicsBrush
|
|
@li wxGraphicsContext
|
|
@li wxGraphicsFont
|
|
@li wxGraphicsMatrix
|
|
@li wxGraphicsPath
|
|
@li wxGraphicsPen
|
|
@li wxIcon
|
|
@li wxImage
|
|
@li wxMetafile
|
|
@li wxPalette
|
|
@li wxPen
|
|
@li wxRegion
|
|
@li wxString
|
|
@li wxVariant
|
|
@li wxVariantData
|
|
|
|
Note that the list above reports the objects which are reference counted in all
|
|
ports of wxWidgets; some ports may use this technique also for other classes.
|
|
|
|
All the objects implement a function @b IsOk() to test if they are referencing valid
|
|
data; when the objects are in uninitialized state, you can only use the @b IsOk() getter;
|
|
trying to call any other getter, e.g. wxBrush::GetStyle() on the ::wxNullBrush object,
|
|
will result in an assert failure in debug builds.
|
|
|
|
|
|
@section overview_refcount_object Making Your Own Reference Counted Class
|
|
|
|
Reference counting can be implemented easily using wxObject or using
|
|
the intermediate wxRefCounter class directly.
|
|
Alternatively, you can also use the wxObjectDataPtr<T> template.
|
|
|
|
First, derive a new class from wxRefCounter (or wxObjectRefData when
|
|
using a wxObject derived class) and put the memory-consuming data in it.
|
|
|
|
Then derive a new class from wxObject and implement there the public interface
|
|
which will be seen by the user of your class. You'll probably want to add a
|
|
function to your class which does the cast from wxObjectRefData to your
|
|
class-specific shared data. For example:
|
|
|
|
@code
|
|
MyClassRefData* GetData() const
|
|
{
|
|
return wx_static_cast(MyClassRefData*, m_refData);
|
|
}
|
|
@endcode
|
|
|
|
In fact, any time you need to read the data from your wxObject-derived class,
|
|
you will need to call this function.
|
|
|
|
@note Any time you need to actually modify the data placed inside your wxObject
|
|
derived class, you must first call the wxObject::UnShare() function to ensure
|
|
that the modifications won't affect other instances which are eventually
|
|
sharing your object's data.
|
|
|
|
*/
|
|
|