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| <a name="Writing-an-Xmethod"></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 <class T> |
| class MyTemplate |
| { |
| public: |
| MyTemplate () : dsize_(10), data_ (new T [10]) { } |
| ~MyTemplate () { delete [] data_; } |
| |
| int footprint (void) |
| { |
| return sizeof (T) * dsize_ + sizeof (MyTemplate<T>); |
| } |
| |
| 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<[ \t\n]*[_a-zA-Z][ _a-zA-Z0-9]*>', |
| 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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| |
