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<h5 class="subsubsection">23.2.2.15 Writing an Xmethod</h5>

<p><a name="index-writing-xmethods-in-Python-2051"></a>
Implementing xmethods in Python will require implementing xmethod
matchers and xmethod workers (see <a href="Xmethods-In-Python.html#Xmethods-In-Python">Xmethods In Python</a>).  Consider
the following C<tt>++</tt> class:

<pre class="smallexample">     class MyClass
     {
     public:
       MyClass (int a) : a_(a) { }
     
       int geta (void) { return a_; }
       int operator+ (int b);
     
     private:
       int a_;
     };
     
     int
     MyClass::operator+ (int b)
     {
       return a_ + b;
     }
</pre>
   <p class="noindent">Let us define two xmethods for the class <code>MyClass</code>, one
replacing the method <code>geta</code>, and another adding an overloaded
flavor of <code>operator+</code> which takes a <code>MyClass</code> argument (the
C<tt>++</tt> code above already has an overloaded <code>operator+</code>
which takes an <code>int</code> argument).  The xmethod matcher can be
defined as follows:

<pre class="smallexample">     class MyClass_geta(gdb.xmethod.XMethod):
         def __init__(self):
             gdb.xmethod.XMethod.__init__(self, 'geta')
     
         def get_worker(self, method_name):
             if method_name == 'geta':
                 return MyClassWorker_geta()
     
     
     class MyClass_sum(gdb.xmethod.XMethod):
         def __init__(self):
             gdb.xmethod.XMethod.__init__(self, 'sum')
     
         def get_worker(self, method_name):
             if method_name == 'operator+':
                 return MyClassWorker_plus()
     
     
     class MyClassMatcher(gdb.xmethod.XMethodMatcher):
         def __init__(self):
             gdb.xmethod.XMethodMatcher.__init__(self, 'MyClassMatcher')
             # List of methods 'managed' by this matcher
             self.methods = [MyClass_geta(), MyClass_sum()]
     
         def match(self, class_type, method_name):
             if class_type.tag != 'MyClass':
                 return None
             workers = []
             for method in self.methods:
                 if method.enabled:
                     worker = method.get_worker(method_name)
                     if worker:
                         workers.append(worker)
     
             return workers
</pre>
   <p class="noindent">Notice that the <code>match</code> method of <code>MyClassMatcher</code> returns
a worker object of type <code>MyClassWorker_geta</code> for the <code>geta</code>
method, and a worker object of type <code>MyClassWorker_plus</code> for the
<code>operator+</code> method.  This is done indirectly via helper classes
derived from <code>gdb.xmethod.XMethod</code>.  One does not need to use the
<code>methods</code> attribute in a matcher as it is optional.  However, if a
matcher manages more than one xmethod, it is a good practice to list the
xmethods in the <code>methods</code> attribute of the matcher.  This will then
facilitate enabling and disabling individual xmethods via the
<code>enable/disable</code> commands.  Notice also that a worker object is
returned only if the corresponding entry in the <code>methods</code> attribute
of the matcher is enabled.

   <p>The implementation of the worker classes returned by the matcher setup
above is as follows:

<pre class="smallexample">     class MyClassWorker_geta(gdb.xmethod.XMethodWorker):
         def get_arg_types(self):
             return None
     
         def get_result_type(self, obj):
             return gdb.lookup_type('int')
     
         def __call__(self, obj):
             return obj['a_']
     
     
     class MyClassWorker_plus(gdb.xmethod.XMethodWorker):
         def get_arg_types(self):
             return gdb.lookup_type('MyClass')
     
         def get_result_type(self, obj):
             return gdb.lookup_type('int')
     
         def __call__(self, obj, other):
             return obj['a_'] + other['a_']
</pre>
   <p>For <span class="sc">gdb</span> to actually lookup a xmethod, it has to be
registered with it.  The matcher defined above is registered with
<span class="sc">gdb</span> globally as follows:

<pre class="smallexample">     gdb.xmethod.register_xmethod_matcher(None, MyClassMatcher())
</pre>
   <p>If an object <code>obj</code> of type <code>MyClass</code> is initialized in C<tt>++</tt>
code as follows:

<pre class="smallexample">     MyClass obj(5);
</pre>
   <p class="noindent">then, after loading the Python script defining the xmethod matchers
and workers into <code>GDBN</code>, invoking the method <code>geta</code> or using
the operator <code>+</code> on <code>obj</code> will invoke the xmethods
defined above:

<pre class="smallexample">     (gdb) p obj.geta()
     $1 = 5
     
     (gdb) p obj + obj
     $2 = 10
</pre>
   <p>Consider another example with a C++ template class:

<pre class="smallexample">     template &lt;class T&gt;
     class MyTemplate
     {
     public:
       MyTemplate () : dsize_(10), data_ (new T [10]) { }
       ~MyTemplate () { delete [] data_; }
     
       int footprint (void)
       {
         return sizeof (T) * dsize_ + sizeof (MyTemplate&lt;T&gt;);
       }
     
     private:
       int dsize_;
       T *data_;
     };
</pre>
   <p>Let us implement an xmethod for the above class which serves as a
replacement for the <code>footprint</code> method.  The full code listing
of the xmethod workers and xmethod matchers is as follows:

<pre class="smallexample">     class MyTemplateWorker_footprint(gdb.xmethod.XMethodWorker):
         def __init__(self, class_type):
             self.class_type = class_type
     
         def get_arg_types(self):
             return None
     
         def get_result_type(self):
             return gdb.lookup_type('int')
     
         def __call__(self, obj):
             return (self.class_type.sizeof +
                     obj['dsize_'] *
                     self.class_type.template_argument(0).sizeof)
     
     
     class MyTemplateMatcher_footprint(gdb.xmethod.XMethodMatcher):
         def __init__(self):
             gdb.xmethod.XMethodMatcher.__init__(self, 'MyTemplateMatcher')
     
         def match(self, class_type, method_name):
             if (re.match('MyTemplate&lt;[ \t\n]*[_a-zA-Z][ _a-zA-Z0-9]*&gt;',
                          class_type.tag) and
                 method_name == 'footprint'):
                 return MyTemplateWorker_footprint(class_type)
</pre>
   <p>Notice that, in this example, we have not used the <code>methods</code>
attribute of the matcher as the matcher manages only one xmethod.  The
user can enable/disable this xmethod by enabling/disabling the matcher
itself.

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