102 lines
5.4 KiB
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102 lines
5.4 KiB
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<title>Xmethods In Python - Debugging with GDB</title>
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<div class="node">
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<a name="Xmethods-In-Python"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Xmethod-API.html#Xmethod-API">Xmethod API</a>,
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Previous: <a rel="previous" accesskey="p" href="Unwinding-Frames-in-Python.html#Unwinding-Frames-in-Python">Unwinding Frames in Python</a>,
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Up: <a rel="up" accesskey="u" href="Python-API.html#Python-API">Python API</a>
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<h5 class="subsubsection">23.2.2.13 Xmethods In Python</h5>
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<p><a name="index-xmethods-in-Python-2039"></a>
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<dfn>Xmethods</dfn> are additional methods or replacements for existing
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methods of a C<tt>++</tt> class. This feature is useful for those cases
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where a method defined in C<tt>++</tt> source code could be inlined or
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optimized out by the compiler, making it unavailable to <span class="sc">gdb</span>.
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For such cases, one can define an xmethod to serve as a replacement
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for the method defined in the C<tt>++</tt> source code. <span class="sc">gdb</span> will
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then invoke the xmethod, instead of the C<tt>++</tt> method, to
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evaluate expressions. One can also use xmethods when debugging
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with core files. Moreover, when debugging live programs, invoking an
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xmethod need not involve running the inferior (which can potentially
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perturb its state). Hence, even if the C<tt>++</tt> method is available, it
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is better to use its replacement xmethod if one is defined.
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<p>The xmethods feature in Python is available via the concepts of an
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<dfn>xmethod matcher</dfn> and an <dfn>xmethod worker</dfn>. To
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implement an xmethod, one has to implement a matcher and a
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corresponding worker for it (more than one worker can be
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implemented, each catering to a different overloaded instance of the
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method). Internally, <span class="sc">gdb</span> invokes the <code>match</code> method of a
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matcher to match the class type and method name. On a match, the
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<code>match</code> method returns a list of matching <em>worker</em> objects.
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Each worker object typically corresponds to an overloaded instance of
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the xmethod. They implement a <code>get_arg_types</code> method which
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returns a sequence of types corresponding to the arguments the xmethod
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requires. <span class="sc">gdb</span> uses this sequence of types to perform
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overload resolution and picks a winning xmethod worker. A winner
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is also selected from among the methods <span class="sc">gdb</span> finds in the
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C<tt>++</tt> source code. Next, the winning xmethod worker and the
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winning C<tt>++</tt> method are compared to select an overall winner. In
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case of a tie between a xmethod worker and a C<tt>++</tt> method, the
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xmethod worker is selected as the winner. That is, if a winning
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xmethod worker is found to be equivalent to the winning C<tt>++</tt>
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method, then the xmethod worker is treated as a replacement for
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the C<tt>++</tt> method. <span class="sc">gdb</span> uses the overall winner to invoke the
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method. If the winning xmethod worker is the overall winner, then
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the corresponding xmethod is invoked via the <code>__call__</code> method
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of the worker object.
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<p>If one wants to implement an xmethod as a replacement for an
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existing C<tt>++</tt> method, then they have to implement an equivalent
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xmethod which has exactly the same name and takes arguments of
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exactly the same type as the C<tt>++</tt> method. If the user wants to
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invoke the C<tt>++</tt> method even though a replacement xmethod is
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available for that method, then they can disable the xmethod.
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<p>See <a href="Xmethod-API.html#Xmethod-API">Xmethod API</a>, for API to implement xmethods in Python.
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See <a href="Writing-an-Xmethod.html#Writing-an-Xmethod">Writing an Xmethod</a>, for implementing xmethods in Python.
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</body></html>
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