qatomicscopedvaluerollback.h

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// Copyright (C) 2022 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
#ifndef QATOMICSCOPEDVALUEROLLBACK_H
#define QATOMICSCOPEDVALUEROLLBACK_H
 
#include <QtCore/qassert.h>
#include <QtCore/qatomic.h>
#include <QtCore/qcompilerdetection.h>
#include <QtCore/qtclasshelpermacros.h>
#include <QtCore/qtconfigmacros.h>
 
#include <atomic>
 
QT_BEGIN_NAMESPACE
 
template <typename T>
class QAtomicScopedValueRollback
{
    std::atomic<T> &m_atomic;
    T m_value;
    std::memory_order m_mo;
 
    Q_DISABLE_COPY_MOVE(QAtomicScopedValueRollback)
 
    static constexpr std::memory_order store_part(std::memory_order mo) noexcept
    {
        switch (mo) {
        case std::memory_order_relaxed:
        case std::memory_order_consume:
        case std::memory_order_acquire: return std::memory_order_relaxed;
        case std::memory_order_release:
        case std::memory_order_acq_rel: return std::memory_order_release;
        case std::memory_order_seq_cst: return std::memory_order_seq_cst;
        }
        // GCC 8.x does not treat __builtin_unreachable() as constexpr
#if !defined(Q_CC_GNU_ONLY) || (Q_CC_GNU >= 900)
        // NOLINTNEXTLINE(qt-use-unreachable-return): Triggers on Clang, breaking GCC 8
        Q_UNREACHABLE();
#endif
        return std::memory_order_seq_cst;
    }
 
    static constexpr std::memory_order load_part(std::memory_order mo) noexcept
    {
        switch (mo) {
        case std::memory_order_relaxed:
        case std::memory_order_release: return std::memory_order_relaxed;
        case std::memory_order_consume: return std::memory_order_consume;
        case std::memory_order_acquire:
        case std::memory_order_acq_rel: return std::memory_order_acquire;
        case std::memory_order_seq_cst: return std::memory_order_seq_cst;
        }
        // GCC 8.x does not treat __builtin_unreachable() as constexpr
#if !defined(Q_CC_GNU_ONLY) || (Q_CC_GNU >= 900)
        // NOLINTNEXTLINE(qt-use-unreachable-return): Triggers on Clang, breaking GCC 8
        Q_UNREACHABLE();
#endif
        return std::memory_order_seq_cst;
    }
public:
    //
    // std::atomic:
    //
    Q_NODISCARD_CTOR
    explicit constexpr
    QAtomicScopedValueRollback(std::atomic<T> &var,
                               std::memory_order mo = std::memory_order_seq_cst)
        : m_atomic(var), m_value(var.load(load_part(mo))), m_mo(mo) {}
 
    Q_NODISCARD_CTOR
    explicit constexpr
    QAtomicScopedValueRollback(std::atomic<T> &var, T value,
                               std::memory_order mo = std::memory_order_seq_cst)
        : m_atomic(var), m_value(var.exchange(value, mo)), m_mo(mo) {}
 
    //
    // Q(Basic)AtomicInteger:
    //
    Q_NODISCARD_CTOR
    explicit constexpr
    QAtomicScopedValueRollback(QBasicAtomicInteger<T> &var,
                               std::memory_order mo = std::memory_order_seq_cst)
        : QAtomicScopedValueRollback(var._q_value, mo) {}
 
    Q_NODISCARD_CTOR
    explicit constexpr
    QAtomicScopedValueRollback(QBasicAtomicInteger<T> &var, T value,
                               std::memory_order mo = std::memory_order_seq_cst)
        : QAtomicScopedValueRollback(var._q_value, value, mo) {}
 
    //
    // Q(Basic)AtomicPointer:
    //
    Q_NODISCARD_CTOR
    explicit constexpr
    QAtomicScopedValueRollback(QBasicAtomicPointer<std::remove_pointer_t<T>> &var,
                               std::memory_order mo = std::memory_order_seq_cst)
        : QAtomicScopedValueRollback(var._q_value, mo) {}
 
    Q_NODISCARD_CTOR
    explicit constexpr
    QAtomicScopedValueRollback(QBasicAtomicPointer<std::remove_pointer_t<T>> &var, T value,
                               std::memory_order mo = std::memory_order_seq_cst)
        : QAtomicScopedValueRollback(var._q_value, value, mo) {}
 
    ~QAtomicScopedValueRollback()
    {
        m_atomic.store(m_value, store_part(m_mo));
    }
 
    void commit()
    {
        m_value = m_atomic.load(load_part(m_mo));
    }
};
 
template <typename T>
QAtomicScopedValueRollback(QBasicAtomicPointer<T> &)
    -> QAtomicScopedValueRollback<T*>;
template <typename T>
QAtomicScopedValueRollback(QBasicAtomicPointer<T> &, std::memory_order)
    -> QAtomicScopedValueRollback<T*>;
 
QT_END_NAMESPACE
 
#endif // QATOMICASCOPEDVALUEROLLBACK_H