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# Boost.Python

## Header <boost/python/def_visitor.hpp>

## Contents

Introduction
Classes
  Class `def_visitor`
    Class `def_visitor` synopsis
    Class `def_visitor` requirements
Example

## Introduction

`<boost/python/def_visitor.hpp>` provides a generic visitation interface through which the class_ def member functionality can be extended non-intrusively to avoid cluttering the class_ interface. It declares the `def_visitor<T>` class template, which is parameterized on the derived type `DerivedVisitor`, which provides the actual def functionality through its visit member functions.

## Classes

### Class template `def_visitor<DerivedVisitor>`

The class def_visitor is a base class paramaterized by its derived class. The def_visitor class is a protocol class. Its derived class, DerivedVisitor, is expected to have a member function visit. The def_visitor class is never instantiated directly. Instead, an instance of its subclass, DerivedVisitor, is passed on as an argument to the class_ def member function.

#### Class `def_visitor` synopsis

    namespace boost { namespace python {

        template <class DerivedVisitor>
        class def_visitor {};
    }

### `def_visitor` requirements

The client supplied class `DerivedVisitor` template parameter is expected to:

- be privately derived from def_visitor
- grant friend access to class def_visitor_access
- define either or both visit member functions listed in the table below:

Expression | Return Type | Requirements | Effects
visitor.visit(cls) | void | cls is an instance of a class_ being wrapped to Python. visitor is a def_visitor derived class. | A call to cls.def(visitor) forwards to this member function.
visitor.visit(cls, name, options) | void | cls is a class_ instance, name is a C string. visitor is a def_visitor derived class. options is a context specific optional argument. | A call to cls.def(name, visitor) or cls.def(name, visitor, options) forwards to this member function.

## Example

    class X {/*...*/};

    class my_def_visitor : boost::python::def_visitor<my_def_visitor>
    {
        friend class def_visitor_access;

        template <class classT>
        void visit(classT& c) const
        {
            c
                .def("foo", &my_def_visitor::foo)
                .def("bar", &my_def_visitor::bar)
            ;
        }

        static void foo(X& self);
        static void bar(X& self);
    };

    BOOST_PYTHON_MODULE(my_ext)
    {
        class_<X>("X")
            .def(my_def_visitor())
        ;
    }

Revised 27 August, 2003

© Copyright Joel de Guzman 2003.