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+<title>Debugging with GDB: Xmethods In Python</title>
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+<a name="Xmethods-In-Python"></a>
+<div class="header">
+<p>
+Next: <a href="Xmethod-API.html#Xmethod-API" accesskey="n" rel="next">Xmethod API</a>, Previous: <a href="Unwinding-Frames-in-Python.html#Unwinding-Frames-in-Python" accesskey="p" rel="previous">Unwinding Frames in Python</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
+</div>
+<hr>
+<a name="Xmethods-In-Python-1"></a>
+<h4 class="subsubsection">23.3.2.13 Xmethods In Python</h4>
+<a name="index-xmethods-in-Python"></a>
+
+<p><em>Xmethods</em> are additional methods or replacements for existing
+methods of a C<tt>++</tt> class.  This feature is useful for those cases
+where a method defined in C<tt>++</tt> source code could be inlined or
+optimized out by the compiler, making it unavailable to <small>GDB</small>.
+For such cases, one can define an xmethod to serve as a replacement
+for the method defined in the C<tt>++</tt> source code.  <small>GDB</small> will
+then invoke the xmethod, instead of the C<tt>++</tt> method, to
+evaluate expressions.  One can also use xmethods when debugging
+with core files.  Moreover, when debugging live programs, invoking an
+xmethod need not involve running the inferior (which can potentially
+perturb its state).  Hence, even if the C<tt>++</tt> method is available, it
+is better to use its replacement xmethod if one is defined.
+</p>
+<p>The xmethods feature in Python is available via the concepts of an
+<em>xmethod matcher</em> and an <em>xmethod worker</em>.  To
+implement an xmethod, one has to implement a matcher and a
+corresponding worker for it (more than one worker can be
+implemented, each catering to a different overloaded instance of the
+method).  Internally, <small>GDB</small> invokes the <code>match</code> method of a
+matcher to match the class type and method name.  On a match, the
+<code>match</code> method returns a list of matching <em>worker</em> objects.
+Each worker object typically corresponds to an overloaded instance of
+the xmethod.  They implement a <code>get_arg_types</code> method which
+returns a sequence of types corresponding to the arguments the xmethod
+requires.  <small>GDB</small> uses this sequence of types to perform
+overload resolution and picks a winning xmethod worker.  A winner
+is also selected from among the methods <small>GDB</small> finds in the
+C<tt>++</tt> source code.  Next, the winning xmethod worker and the
+winning C<tt>++</tt> method are compared to select an overall winner.  In
+case of a tie between a xmethod worker and a C<tt>++</tt> method, the
+xmethod worker is selected as the winner.  That is, if a winning
+xmethod worker is found to be equivalent to the winning C<tt>++</tt>
+method, then the xmethod worker is treated as a replacement for
+the C<tt>++</tt> method.  <small>GDB</small> uses the overall winner to invoke the
+method.  If the winning xmethod worker is the overall winner, then
+the corresponding xmethod is invoked via the <code>__call__</code> method
+of the worker object.
+</p>
+<p>If one wants to implement an xmethod as a replacement for an
+existing C<tt>++</tt> method, then they have to implement an equivalent
+xmethod which has exactly the same name and takes arguments of
+exactly the same type as the C<tt>++</tt> method.  If the user wants to
+invoke the C<tt>++</tt> method even though a replacement xmethod is
+available for that method, then they can disable the xmethod.
+</p>
+<p>See <a href="Xmethod-API.html#Xmethod-API">Xmethod API</a>, for API to implement xmethods in Python.
+See <a href="Writing-an-Xmethod.html#Writing-an-Xmethod">Writing an Xmethod</a>, for implementing xmethods in Python.
+</p>
+<hr>
+<div class="header">
+<p>
+Next: <a href="Xmethod-API.html#Xmethod-API" accesskey="n" rel="next">Xmethod API</a>, Previous: <a href="Unwinding-Frames-in-Python.html#Unwinding-Frames-in-Python" accesskey="p" rel="previous">Unwinding Frames in Python</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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