forward_value.hpp

// Copyright Take Vos 2021.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#include <type_traits>
#include <string>
#include <string_view>
 
#pragma once
 
namespace tt {
 
template<typename T>
struct forward_value {
    using type = std::remove_cvref_t<T>;
 
    [[nodiscard]] constexpr T &&operator()(std::remove_reference_t<T> &t) const noexcept
    {
        return static_cast<T &&>(t);
    }
 
    [[nodiscard]] constexpr T &&operator()(std::remove_reference_t<T> &&t) const noexcept
    {
        static_assert(!std::is_lvalue_reference_v<T>, "Can not forward an rvalue as an lvalue.");
        return static_cast<T &&>(t);
    }
};
 
template<size_t N>
struct forward_value<char const (&)[N]> {
    using type = char const *;
 
    [[nodiscard]] constexpr char const *operator()(char const (&t)[N]) const noexcept
    {
        return static_cast<char const *>(t);
    }
};
 
template<>
struct forward_value<std::string_view> {
    using type = std::string;
 
    [[nodiscard]] std::string operator()(std::string_view t) const noexcept
    {
        return std::string{t};
    }
};
 
template<typename T>
using forward_value_t = typename forward_value<T>::type;
 
} // namespace tt