auto_constant

auto_constant.cpp

#include <iostream>

// impl ----------

template <auto V>
struct auto_constant {
    static constexpr auto value = V;
    
    using value_type = decltype(value);
    using type = auto_constant;
    
    constexpr operator value_type() noexcept { return value; }
    constexpr value_type operator()() const noexcept { return value; }
};

template <auto V>
constexpr auto auto_constant_v = auto_constant<V>::value;

// usage ----------

constexpr auto foo() { return "hoge"; }

int main()
{
    auto_constant<123> a;
    std::cout << a() << std::endl;
    std::cout << auto_constant_v<foo>() << std::endl;
    return 0;
}