Debugging with GDB: Xmethods In Python

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#### 23.3.2.13 Xmethods In Python

Xmethods are additional methods or replacements for existing methods of a C`++` class. This feature is useful for those cases where a method defined in C`++` source code could be inlined or optimized out by the compiler, making it unavailable to GDB. For such cases, one can define an xmethod to serve as a replacement for the method defined in the C`++` source code. GDB will then invoke the xmethod, instead of the C`++` 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`++` method is available, it is better to use its replacement xmethod if one is defined.

The xmethods feature in Python is available via the concepts of an xmethod matcher and an xmethod worker. 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, GDB invokes the `match` method of a matcher to match the class type and method name. On a match, the `match` method returns a list of matching worker objects. Each worker object typically corresponds to an overloaded instance of the xmethod. They implement a `get_arg_types` method which returns a sequence of types corresponding to the arguments the xmethod requires. GDB uses this sequence of types to perform overload resolution and picks a winning xmethod worker. A winner is also selected from among the methods GDB finds in the C`++` source code. Next, the winning xmethod worker and the winning C`++` method are compared to select an overall winner. In case of a tie between a xmethod worker and a C`++` 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`++` method, then the xmethod worker is treated as a replacement for the C`++` method. GDB 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 `__call__` method of the worker object.

If one wants to implement an xmethod as a replacement for an existing C`++` 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`++` method. If the user wants to invoke the C`++` method even though a replacement xmethod is available for that method, then they can disable the xmethod.

See Xmethod API, for API to implement xmethods in Python. See Writing an Xmethod, for implementing xmethods in Python.

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