compile_time_optional.hpp

// Copyright (C) 2022 by Camden Mannett.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#pragma once
 
#include <utility>
 
namespace arg_router::utility
{
template <typename T>
class compile_time_optional
{
public:
    using value_type = T;
 
    constexpr static bool empty = false;
 
    explicit constexpr compile_time_optional(value_type val) noexcept : val_(std::move(val)) {}
 
    constexpr explicit operator bool() const { return !empty; }
 
    constexpr const value_type* operator->() const noexcept { return &val_; }
 
    constexpr const value_type& operator*() const noexcept { return val_; }
 
    value_type* operator->() noexcept { return &val_; }
 
    value_type& operator*() noexcept { return val_; }
 
private:
    value_type val_;
};
 
template <typename T>
class compile_time_optional<std::reference_wrapper<T>>
{
public:
    using value_type = T;
 
    constexpr static bool empty = false;
 
    constexpr explicit operator bool() const { return !empty; }
 
    explicit compile_time_optional(std::reference_wrapper<T> val) noexcept : ref_(val) {}
 
    const value_type* operator->() const noexcept { return &(ref_.get()); }
 
    const value_type& operator*() const noexcept { return ref_; }
 
    value_type* operator->() noexcept { return &(ref_.get()); }
 
    value_type& operator*() noexcept { return ref_; }
 
private:
    std::reference_wrapper<value_type> ref_;
};
 
template <>
class compile_time_optional<void>
{
public:
    constexpr static bool empty = true;
 
    explicit constexpr operator bool() const { return !empty; }
};
 
// Deduction guide
template <typename T = void>
compile_time_optional() -> compile_time_optional<void>;
}  // namespace arg_router::utility