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<title>Writing an Xmethod - Debugging with GDB</title>
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Previous:&nbsp;<a rel="previous" accesskey="p" href="Xmethod-API.html#Xmethod-API">Xmethod API</a>,
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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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