const_v<> template variable

const_v.hpp

// Helper template
template <auto V>
struct const_value
{
	static constexpr auto value = V;

	constexpr operator decltype(V)() const noexcept
	{
		return V;
	}

	constexpr decltype(V) operator ()() const
	{
		return V;
	}
};

// Specialize for function pointers
template <typename R, typename... Args, R(*F)(Args...)>
struct const_value<F>
{
	static constexpr auto value = F;

	constexpr operator decltype(F)() const noexcept
	{
		return F;
	}

	constexpr R operator ()(Args&&... args) const
	{
		return F(std::forward<Args>(args)...);
	}
};

// Specialize for pointer to member function
template <typename R, typename B, typename... Args, R(B::*M)(Args...)>
struct const_value<M>
{
	static constexpr auto value = M;

	constexpr operator decltype(M)() const noexcept
	{
		return M;
	}

	constexpr R operator ()(B& base, Args&&... args) const
	{
		return (base.*M)(std::forward<Args>(args)...);
	}
};

// Specialize for pointer to const member function
template <typename R, typename B, typename... Args, R(B::*M)(Args...) const>
struct const_value<M>
{
	static constexpr auto value = M;

	constexpr operator decltype(M)() const noexcept
	{
		return M;
	}

	constexpr R operator ()(const B& base, Args&&... args) const
	{
		return (base.*M)(std::forward<Args>(args)...);
	}
};

// Specialize for pointer to data member
template <typename R, typename B, R B::* M>
struct const_value<M>
{
	static constexpr auto value = M;

	constexpr operator decltype(M)() const noexcept
	{
		return M;
	}

	constexpr R operator ()(const B& base) const
	{
		return (base.*M);
	}

	constexpr decltype(auto) operator ()(B& base, R value) const
	{
		return (base.*M) = std::move(value);
	}
};

// Bind a template argument (any compatible constant) to a type (callable, convertible)
template <auto V>
constexpr const_value<V> const_v{};