cref_wrapper.hpp

/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
 
// SPDX-FileCopyrightText: 2024 - 2025 Kohei Yoshida
//
// SPDX-License-Identifier: MIT
 
#pragma once
 
#include <functional>
#include <type_traits>
 
namespace mdds { namespace detail {

template<typename T>
class cref_wrapper
{
    std::reference_wrapper<std::add_const_t<T>> m_value;
 
public:
    cref_wrapper(const T& v) noexcept : m_value(std::cref(v))
    {}
 
    const T& get() const noexcept
    {
        return m_value;
    }
 
    bool operator==(const cref_wrapper& r) const
    {
        return get() == r.get();
    }
 
    bool operator!=(const cref_wrapper& r) const
    {
        return !operator==(r);
    }
 
    struct hash
    {
        std::size_t operator()(const cref_wrapper& v) const noexcept(noexcept(std::hash<T>{}(v.get())))
        {
            // NB: hash value must be based on the wrapped value, else two
            // identical values located in different memory locations may end up
            // in different buckets, and the lookup may fail.
            return std::hash<T>{}(v.get());
        }
    };
};
 
}} // namespace mdds::detail
 
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