###

# Boost.Python

## Header <call_method.hpp>

## Contents

Introduction
Functions
  call_method
Example(s)

## Introduction

`<boost/python/call_method.hpp>` defines the `call_method` family of overloaded function templates, used to invoke callable attributes of Python objects from C++.

## Functions

    template <class R, class A1, class A2, ... class An>
    R call_method(PyObject* self, char const* method, A1 const&, A2 const&, ... An const&)

Requires: `R` is a pointer type, reference type, or a complete type with an accessible copy constructor
Effects: Invokes `self.method(a1, a2, ...an)` in Python, where `a1`...`an` are the arguments to `call_method()`, converted to Python objects. For a complete semantic description, see this page.
Returns: The result of the Python call, converted to the C++ type `R`.
Rationale: `call_method` is critical to implementing C++ virtual functions which are overridable in Python, as shown by the example below.

## Example(s)

The following C++ illustrates the use of `call_method` in wrapping a class with a virtual function that can be overridden in Python:

### C++ Module Definition

    #include <boost/python/module.hpp>
    #include <boost/python/class.hpp>
    #include <boost/utility.hpp>
    #include <cstring>

    // class to be wrapped
    class Base
    {
     public:
       virtual char const* class_name() const { return "Base"; }
       virtual ~Base();
    };

    bool is_base(Base* b)
    {
       return !std::strcmp(b->class_name(), "Base");
    }

    // Wrapper code begins here
    using namespace boost::python;

    // Callback class
    class Base_callback : public Base
    {
     public:
       Base_callback(PyObject* self) : m_self(self) {}

       char const* class_name() const { return call_method<char const*>(m_self, "class_name"); }
       char const* Base_name() const { return Base::class_name(); }
     private:
       PyObject* const m_self;
    };

    using namespace boost::python;
    BOOST_PYTHON_MODULE(my_module)
    {
        def("is_base", is_base);

        class_<Base,Base_callback, noncopyable>("Base")
            .def("class_name", &Base_callback::Base_name)
            ;

    }

### Python Code

    >>> from my_module import *
    >>> class Derived(Base):
    ...    def __init__(self):
    ...       Base.__init__(self)
    ...    def class_name(self):
    ...       return self.__class__.__name__
    ...
    >>> is_base(Base()) # calls the class_name() method from C++
    1
    >>> is_base(Derived())
    0

Revised 13 November, 2002

© Copyright Dave Abrahams 2002.