d479632a8f
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42561 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
63 lines
2.6 KiB
TeX
63 lines
2.6 KiB
TeX
\section{Reference counting}\label{trefcount}
|
|
|
|
\subsection{Reference counting and why you shouldn't care about it}\label{refcount}
|
|
|
|
Many wxWidgets objects use a technique known as \it{reference counting}, also known
|
|
as {\it copy on write} (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.
|
|
|
|
\subsection{List of reference-counted wxWidgets classes}\label{refcountlist}
|
|
|
|
The following classes in wxWidgets have efficient (i.e. fast) assignment operators
|
|
and copy constructors since they are reference-counted:
|
|
|
|
\helpref{wxAcceleratorTable}{wxacceleratortable}\\
|
|
\helpref{wxBrush}{wxbrush}\\
|
|
\helpref{wxCursor}{wxcursor}\\
|
|
\helpref{wxFont}{wxfont}\\
|
|
\helpref{wxImage}{wximage}\\
|
|
\helpref{wxMetafile}{wxmetafile}\\
|
|
\helpref{wxPalette}{wxpalette}\\
|
|
\helpref{wxPen}{wxpen}\\
|
|
\helpref{wxRegion}{wxregion}\\
|
|
\helpref{wxRegionIterator}{wxregioniterator}\\
|
|
\helpref{wxString}{wxstring}
|
|
|
|
\subsection{Make your own reference-counted class}\label{wxobjectoverview}
|
|
|
|
Reference counting can be implemented easily using \helpref{wxObject}{wxobject}
|
|
and \helpref{wxObjectRefData}{wxobjectrefdata} classes.
|
|
|
|
First, derive a new class from \helpref{wxObjectRefData}{wxobjectrefdata} and
|
|
put there the memory-consuming data.
|
|
|
|
Then derive a new class from \helpref{wxObject}{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
|
|
\helpref{wxObjectRefData}{wxobjectrefdata} to your class-specific shared data; e.g.:
|
|
|
|
\begin{verbatim}
|
|
MyClassRefData *GetData() const { return wx_static_cast(MyClassRefData*, m_refData); }
|
|
\end{verbatim}
|
|
|
|
in fact, all times you'll need to read the data from your wxObject-derived class,
|
|
you'll need to call such function.
|
|
|
|
Very important, all times you need to actually modify the data placed inside your
|
|
wxObject-derived class, you must first call the wxObject::UnShare
|
|
function to be sure that the modifications won't affect other instances which are
|
|
eventually sharing your object's data.
|
|
|