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## Chapter 1. Boost.Functional/OverloadedFunction 1.0.0

### Lorenzo Caminiti `<lorcaminiti@gmail.com>`

Copyright © 2011, 2012 Lorenzo Caminiti

Distributed under the Boost Software License, Version 1.0 (see accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

This library allows to overload different functions into a single function object.

## Introduction

Consider the following functions which have distinct signatures:

    const std::string& identity_s(const std::string& x) // Function (as pointer).
        { return x; }

    int identity_i_impl(int x) { return x; }
    int (&identity_i)(int) = identity_i_impl; // Function reference.

    double identity_d_impl(double x) { return x; }
    boost::function<double (double)> identity_d = identity_d_impl; // Functor.

Instead of calling them using their separate names (here `BOOST_TEST` is equivalent to `assert`): [1]

    BOOST_TEST(identity_s("abc") == "abc");
    BOOST_TEST(identity_i(123) == 123);
    BOOST_TEST(identity_d(1.23) == 1.23);

It is possible to use this library to create a single overloaded function object (or functor) named `identity` that aggregates together the calls to the specific functions (see also `functor.cpp` and `identity.hpp`):

    boost::overloaded_function<
          const std::string& (const std::string&)
        , int (int)
        , double (double)
    > identity(identity_s, identity_i, identity_d);

    // All calls via single `identity` function.
    BOOST_TEST(identity("abc") == "abc");
    BOOST_TEST(identity(123) == 123);
    BOOST_TEST(identity(1.23) == 1.23);

Note how the functions are called via a single overloaded function object `identity` instead of using their different names `identity_s`, `identity_i`, and `identity_d`.

[1] In most of the examples presented in this documentation, the Boost.Detail/LightweightTest (`boost/detail/lightweight_test.hpp`) macro `BOOST_TEST` is used to check correctness conditions (conceptually similar to `assert`). A failure of the checked condition does not abort the execution of the program, it will instead make `boost::report_errors` return a non-zero program exit code. Using Boost.Detail/LightweightTest allows to add the examples to the library regression tests so to make sure that they always compile and run correctly.

Last revised: December 14, 2017 at 00:00:17 GMT |