Restrict creation of string_view to string literals only

LiteralStringViewHolder.h

#ifndef LITERALSTRINGVIEW__H__
#define LITERALSTRINGVIEW__H__

#include <cassert>
#include <string_view>



class LiteralStringViewHolder;



namespace literals
{
namespace string_view_literals
{

constexpr
LiteralStringViewHolder operator"" _lit_sv(const char* p, size_t n) noexcept;

} // namespace string_view_literals
} // namespace literals



class LiteralStringViewHolder
{
public:
	constexpr LiteralStringViewHolder() noexcept = default;

#if defined(__x86_64__) || defined(_M_X64)
	[[nodiscard]] constexpr std::string_view view() const noexcept
	{
		return { data(), size() };
	}
#else
	[[nodiscard]] constexpr std::string_view view() const noexcept
	{
		return { p, n };
	}
#endif // #if defined(__x86_64__) || defined(_M_X64)

private:
	friend constexpr LiteralStringViewHolder literals::string_view_literals::
		operator"" _lit_sv(const char* p, size_t n) noexcept;


#if defined(__x86_64__) || defined(_M_X64)

	constexpr static uintptr_t pointer_mask = (1ull << 48) - 1;
	constexpr static const char* zero_pointer = nullptr;

	constexpr static uintptr_t pack_pointer(const char* p) noexcept
	{
		return static_cast<uintptr_t>(p - zero_pointer) & pointer_mask;
	}

	constexpr static uintptr_t pack_mask(size_t n) noexcept
	{
		return static_cast<uintptr_t>(n & 0xFFFF) << 48;
	}

	constexpr const char* data() const noexcept
	{
		return zero_pointer + (packed & pointer_mask);
	}

	constexpr size_t size() const noexcept
	{
		return static_cast<size_t>(packed >> 48);
	}

	constexpr LiteralStringViewHolder(const char* p, size_t n) noexcept
		: packed(pack_pointer(p) | pack_mask(n))
	{
	}
	
	uintptr_t packed = 0;

#else
	constexpr LiteralStringViewHolder(const char* p, size_t n) noexcept
		: p(p), n(n)
	{
	}
	const char* p = nullptr;
	size_t n = 0;

#endif // #if defined(__x86_64__) || defined(_M_X64)
};



namespace literals
{
namespace string_view_literals
{

constexpr
LiteralStringViewHolder operator"" _lit_sv(const char* p, size_t n) noexcept
{
	assert(n < 0xFFFF);
	return LiteralStringViewHolder(p, n);
}

} // namespace string_view_literals
} // namespace literals

#endif