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  18. <title>Xmethods In Python (Debugging with GDB)</title>

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  63. <a name="Xmethods-In-Python"></a>

  64. <div class="header">

  65. <p>

  66. 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="prev">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>

  67. </div>

  68. <hr>

  69. <a name="Xmethods-In-Python-1"></a>

  70. <h4 class="subsubsection">23.2.2.13 Xmethods In Python</h4>

  71. <a name="index-xmethods-in-Python"></a>

  72. 

  73. <p><em>Xmethods</em> are additional methods or replacements for existing

  74. methods of a C<tt>++</tt> class.  This feature is useful for those cases

  75. where a method defined in C<tt>++</tt> source code could be inlined or

  76. optimized out by the compiler, making it unavailable to <small>GDB</small>.

  77. For such cases, one can define an xmethod to serve as a replacement

  78. for the method defined in the C<tt>++</tt> source code.  <small>GDB</small> will

  79. then invoke the xmethod, instead of the C<tt>++</tt> method, to

  80. evaluate expressions.  One can also use xmethods when debugging

  81. with core files.  Moreover, when debugging live programs, invoking an

  82. xmethod need not involve running the inferior (which can potentially

  83. perturb its state).  Hence, even if the C<tt>++</tt> method is available, it

  84. is better to use its replacement xmethod if one is defined.

  85. </p>

  86. <p>The xmethods feature in Python is available via the concepts of an

  87. <em>xmethod matcher</em> and an <em>xmethod worker</em>.  To

  88. implement an xmethod, one has to implement a matcher and a

  89. corresponding worker for it (more than one worker can be

  90. implemented, each catering to a different overloaded instance of the

  91. method).  Internally, <small>GDB</small> invokes the <code>match</code> method of a

  92. matcher to match the class type and method name.  On a match, the

  93. <code>match</code> method returns a list of matching <em>worker</em> objects.

  94. Each worker object typically corresponds to an overloaded instance of

  95. the xmethod.  They implement a <code>get_arg_types</code> method which

  96. returns a sequence of types corresponding to the arguments the xmethod

  97. requires.  <small>GDB</small> uses this sequence of types to perform

  98. overload resolution and picks a winning xmethod worker.  A winner

  99. is also selected from among the methods <small>GDB</small> finds in the

  100. C<tt>++</tt> source code.  Next, the winning xmethod worker and the

  101. winning C<tt>++</tt> method are compared to select an overall winner.  In

  102. case of a tie between a xmethod worker and a C<tt>++</tt> method, the

  103. xmethod worker is selected as the winner.  That is, if a winning

  104. xmethod worker is found to be equivalent to the winning C<tt>++</tt>

  105. method, then the xmethod worker is treated as a replacement for

  106. the C<tt>++</tt> method.  <small>GDB</small> uses the overall winner to invoke the

  107. method.  If the winning xmethod worker is the overall winner, then

  108. the corresponding xmethod is invoked via the <code>__call__</code> method

  109. of the worker object.

  110. </p>

  111. <p>If one wants to implement an xmethod as a replacement for an

  112. existing C<tt>++</tt> method, then they have to implement an equivalent

  113. xmethod which has exactly the same name and takes arguments of

  114. exactly the same type as the C<tt>++</tt> method.  If the user wants to

  115. invoke the C<tt>++</tt> method even though a replacement xmethod is

  116. available for that method, then they can disable the xmethod.

  117. </p>

  118. <p>See <a href="Xmethod-API.html#Xmethod-API">Xmethod API</a>, for API to implement xmethods in Python.

  119. See <a href="Writing-an-Xmethod.html#Writing-an-Xmethod">Writing an Xmethod</a>, for implementing xmethods in Python.

  120. </p>

  121. <hr>

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  124. 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="prev">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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