TheRock/rocm-systems/projects/rocprof-trace-decoder/source/cow_ptr.hpp

// MIT License
//
// Copyright (c) 2026 Advanced Micro Devices, Inc. All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#pragma once

#include <cstddef>
#include <memory>
#include <new>
#include <utility>

template <typename T> class CowPtr
{
public:
    CowPtr() = default;

    const T& read() const
    {
        if (!ptr_) return empty();
        return *ptr_;
    }

    T& write()
    {
        if (!ptr_)
            ptr_ = std::make_shared<T>();
        else if (ptr_.use_count() > 1)
            ptr_ = std::make_shared<T>(*ptr_);
        return const_cast<T&>(*ptr_);
    }

    bool null() const { return !ptr_; }

private:
    static const T& empty()
    {
        alignas(T) static std::byte storage[sizeof(T)];
        static const T* value = ::new (static_cast<void*>(storage)) T{};
        return *value;
    }

    std::shared_ptr<const T> ptr_{};
};