common_type.hpp

#ifndef BOOST_TYPE_TRAITS_COMMON_TYPE_HPP_INCLUDED
#define BOOST_TYPE_TRAITS_COMMON_TYPE_HPP_INCLUDED

//
//  Copyright 2015 Peter Dimov
//
//  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
//

#include <boost/type_traits/decay.hpp>
#include <boost/utility/declval.hpp>

#if defined(BOOST_NO_CXX11_DECLTYPE)
#include "common_type_impl.hpp"
#endif

namespace boost
{

// variadic common_type

#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)

template<class... T> struct common_type
{
};

template<class T1, class T2, class... T>
struct common_type<T1, T2, T...>: common_type<typename common_type<T1, T2>::type, T...>
{
};

#else

template<
    class T1 = void, class T2 = void, class T3 = void,
    class T4 = void, class T5 = void, class T6 = void,
    class T7 = void, class T8 = void, class T9 = void
>
struct common_type: common_type<typename common_type<T1, T2>::type, T3, T4, T5, T6, T7, T8, T9>
{
};

#endif // !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)

// one argument

template<class T> struct common_type<T>: boost::decay<T>
{
};

// two arguments

namespace type_traits_detail
{

// binary common_type

#if !defined(BOOST_NO_CXX11_DECLTYPE)

template<class T1, class T2> struct common_type_impl: boost::decay<decltype( boost::declval<bool>()? boost::declval<T1>(): boost::declval<T2>() )>
{
};

#endif // #if !defined(BOOST_NO_CXX11_DECLTYPE)

// decay helper

template<class T1, class T2, class T1d = typename boost::decay<T1>::type, class T2d = typename boost::decay<T2>::type> struct common_type_decay_helper: boost::common_type<T1d, T2d>
{
};

template<class T1, class T2> struct common_type_decay_helper<T1, T2, T1, T2>: common_type_impl<T1, T2>
{
};

} // type_traits_detail

template<class T1, class T2> struct common_type<T1, T2>: type_traits_detail::common_type_decay_helper<T1, T2>
{
};

} // namespace boost

#endif // #ifndef BOOST_TYPE_TRAITS_COMMON_TYPE_HPP_INCLUDED