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| <h5 class="subsubsection">23.2.2.13 Xmethods In Python</h5> |
| |
| <p><a name="index-xmethods-in-Python-2039"></a> |
| <dfn>Xmethods</dfn> 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 <span class="sc">gdb</span>. |
| For such cases, one can define an xmethod to serve as a replacement |
| for the method defined in the C<tt>++</tt> source code. <span class="sc">gdb</span> 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>The xmethods feature in Python is available via the concepts of an |
| <dfn>xmethod matcher</dfn> and an <dfn>xmethod worker</dfn>. 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, <span class="sc">gdb</span> 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. <span class="sc">gdb</span> uses this sequence of types to perform |
| overload resolution and picks a winning xmethod worker. A winner |
| is also selected from among the methods <span class="sc">gdb</span> 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. <span class="sc">gdb</span> 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>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>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. |
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