Pure/Common.h at master · splinterofchaos/Pure · GitHub

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#include <utility>
#include <functional>

#pragma once

namespace pure {

using std::declval;
using std::forward;
using std::move;

template< class X > using Decay = typename std::decay<X>::type;

template< class ...X >
using CommonType = Decay<typename std::common_type<X...>::type>;

template< class X, class R >
using ERVal = typename std::enable_if <
    std::is_rvalue_reference<X>::value, R
>::type;
template< class X, class R >
using XRVal = typename std::enable_if <
    not std::is_rvalue_reference< X >::value, R
>::type;

template< class A, class B, class R >
using ESame = typename std::enable_if< std::is_same<A,B>::value, R >::type;
template< class A, class B, class R >
using XSame = typename std::enable_if< !std::is_same<A,B>::value, R >::type;

/* The result of a function applied against X. */
template< class F, class ...X >
using Result = Decay<decltype( declval<F>()( declval<X>()... ) )>;

template< class F, class ...X >
using DResult = Decay<decltype( declval<F>()( declval<Decay<X>>()... ) )>;

constexpr struct Id {
    template< class X >
    constexpr X operator () ( X&& x ) { return forward<X>(x); }

    template< class F, class ...X >
    constexpr auto operator () ( F&& f, X&& ...x )
        -> decltype( forward<F>(f)( forward<X>(x)... ) )
    {
        return forward<F>(f)( forward<X>(x)... );
    }
} id{};

}