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

This is the documentation for an old version of Boost. Click here to view this page for the latest version.


// Copyright (C) 2007, 2008 Steven Watanabe, Joseph Gauterin, Niels Dekker
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// For more information, see


// Note: the implementation of this utility contains various workarounds:
// - swap_impl is put outside the boost namespace, to avoid infinite
// recursion (causing stack overflow) when swapping objects of a primitive
// type.
// - swap_impl has a using-directive, rather than a using-declaration,
// because some compilers (including MSVC 7.1, Borland 5.9.3, and
// Intel 8.1) don't do argument-dependent lookup when it has a
// using-declaration instead.
// - boost::swap has two template arguments, instead of one, to
// avoid ambiguity when swapping objects of a Boost type that does
// not have its own boost::swap overload.

#include <boost/core/enable_if.hpp>
#include <boost/config.hpp>
#include <utility> //for std::swap (C++11)
#include <algorithm> //for std::swap (C++98)
#include <cstddef> //for std::size_t

namespace boost_swap_impl
  // we can't use type_traits here

  template<class T> struct is_const { enum _vt { value = 0 }; };
  template<class T> struct is_const<T const> { enum _vt { value = 1 }; };

  template<class T>
  void swap_impl(T& left, T& right)
    using namespace std;//use std::swap if argument dependent lookup fails

  template<class T, std::size_t N>
  void swap_impl(T (& left)[N], T (& right)[N])
    for (std::size_t i = 0; i < N; ++i)
      ::boost_swap_impl::swap_impl(left[i], right[i]);

namespace boost
  template<class T1, class T2>
  typename enable_if_c< !boost_swap_impl::is_const<T1>::value && !boost_swap_impl::is_const<T2>::value >::type
  swap(T1& left, T2& right)
    ::boost_swap_impl::swap_impl(left, right);