Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Boost.Python

Header <boost/python/implicit.hpp>

Contents

Introduction
Functions
Function Template implicitly_convertible
Example

Introduction

implicitly_convertible allows Boost.Python to implicitly take advantage of a C++ implicit or explicit conversion when matching Python objects to C++ argument types.

Functions

Function template implicitly_convertible

template <class Source, class Target>
void implicitly_convertible();
implicitly_convertible template parameters
Parameter Description
Source The source type of the implicit conversion
Target The target type of the implicit conversion
Requires: The declaration Target t(s);, where s is of type Source, is valid.
Effects: registers an rvalue from_python converter to Target which can succeed for any PyObject* p iff there exists any registered converter which can produce Source rvalues
Rationale: C++ users expect to be able to take advantage of the same sort of interoperability in Python as they do in C++.

Example

C++ module definition

#include <boost/python/class.hpp>
#include <boost/python/implicit.hpp>
#include <boost/python/module.hpp>

using namespace boost::python;

struct X
{
    X(int x) : v(x) {}
    operator int() const { return v; }
    int v;
};

int x_value(X const& x)
{
    return x.v;
}

X make_x(int n) { return X(n); }

BOOST_PYTHON_MODULE(implicit_ext)
{
    def("x_value", x_value);
    def("make_x", make_x);

    class_<X>("X", 
        init<int>())
        ;

    implicitly_convertible<X,int>();
    implicitly_convertible<int,X>();
}

Python code

>>> from implicit_ext import *
>>> x_value(X(42))
42
>>> x_value(42)
42
>>> x = make_x(X(42))
>>> x_value(x)
42

Revised 13 November, 2002

© Copyright Dave Abrahams 2002.