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]] type operator()(T const &t) const noexcept
    {
        return t;
    }
};
 
 
#define MAKE_FORWARD_VALUE(TEMPLATE_TYPE, RETURN_TYPE, ARGUMENT_TYPE) \
    template<> \
    struct forward_value<TEMPLATE_TYPE> { \
        using type = RETURN_TYPE; \
\
        [[nodiscard]] type operator()(ARGUMENT_TYPE t) const noexcept \
        { \
            return type{t}; \
        } \
    };
 
// Copy string_view by string value.
MAKE_FORWARD_VALUE(std::string_view, std::string, std::string_view const &)
MAKE_FORWARD_VALUE(std::string_view const, std::string, std::string_view const &)
MAKE_FORWARD_VALUE(std::string_view &, std::string, std::string_view const &)
MAKE_FORWARD_VALUE(std::string_view const &, std::string, std::string_view const &)
 
// Copy char pointers by string value.
MAKE_FORWARD_VALUE(char *, std::string, char const *)
MAKE_FORWARD_VALUE(char const *, std::string, char const *)
MAKE_FORWARD_VALUE(char * const, std::string, char const *)
MAKE_FORWARD_VALUE(char const * const, std::string, char const *)
MAKE_FORWARD_VALUE(char *&, std::string, char const *)
MAKE_FORWARD_VALUE(char const *&, std::string, char const *)
MAKE_FORWARD_VALUE(char *const&, std::string, char const *)
MAKE_FORWARD_VALUE(char const *const&, std::string, char const *)
 
#undef MAKE_FORWARD_VALUE
 
// Copy string literal by pointer.
template<size_t N>
struct forward_value<char const (&)[N]> {
    using type = char const *;
 
    [[nodiscard]] constexpr type operator()(char const (&t)[N]) const noexcept
    {
        return static_cast<char const *>(t);
    }
};
 
template<typename T>
using forward_value_t = typename forward_value<T>::type;
 
} // namespace tt