qatomicscopedvaluerollback.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
QT_BEGIN_NAMESPACE
 
/*!
    \class QAtomicScopedValueRollback
    \inmodule QtCore
    \brief Provides a QScopedValueRollback for atomic variables.
    \ingroup misc
    \ingroup tools
    \since 6.7
 
    The QAtomicScopedValueRollback class resets an atomic variable to its
    prior value on destruction. It can be used to revert state when an
    exception is thrown without the need to write try-catch blocks.
 
    It can also be used to manage variables that are temporarily set, such as
    reentrancy guards. By using this class, the variable will be reset whether the
    function is exited normally, exited early by a return statement, or exited by
    an exception.
 
    The class works on std::atomic and the Qt atomic classes: QBasicAtomicInteger,
    \l QAtomicInteger, \l QAtomicInt, QBasicAtomicPointer and \l QAtomicPointer.
 
    \target Memory Order
    The memory accesses to the atomic variable \a var are specified using the value
    of \c mo. The memory order follows this mapping:
    \br
    \list
    \li When writing to the atomic variable:
        \list
        \li An acquire ordering performs a relaxed operation instead.
        \li A hybrid acquire-release ordering performs a release operation instead.
        \endlist
    \li When reading from the atomic variable:
        \list
        \li A release ordering performs a relaxed operation instead.
        \li A consume ordering performs a consume operation.
        \li A hybrid acquire-release ordering performs an acquire operation instead.
        \endlist
    \endlist
    \br
    Otherwise, the default memory order is sequential consistent ordering.
 
    \note You should never name the template arguments explicitly, but exclusively
    use Class Template Argument Deduction (CTAD) and let the compiler pick the
    template argument.
 
    \note There is a chance that other threads modify the variable too, which means
    you may lose updates performed by other threads between the call to the
    QAtomicScopedValueRollback constructor and commit() or between commit() and the
    destructor.
 
    \sa QScopedValueRollback
*/
 
/*!
    \fn template <typename T> QAtomicScopedValueRollback<T>::QAtomicScopedValueRollback(std::atomic<T> &var, std::memory_order mo = std::memory_order_seq_cst)
    \fn template <typename T> QAtomicScopedValueRollback<T>::QAtomicScopedValueRollback(QBasicAtomicInteger<T> &var, std::memory_order mo = std::memory_order_seq_cst)
    \fn template <typename T> QAtomicScopedValueRollback<T>::QAtomicScopedValueRollback(QBasicAtomicPointer<std::remove_pointer_t<T>> &var, std::memory_order mo = std::memory_order_seq_cst)
 
    Records the value of \a var in order to restore it on destruction.
 
    This is equivalent to:
    \code
    T old_value = var.load(mo);
    // And in the destructor: var.store(old_value, mo);
    \endcode
    The \c{mo} adjustment for the load is described in the \l {Memory Order} section.
*/
 
/*!
    \fn template <typename T> QAtomicScopedValueRollback<T>::QAtomicScopedValueRollback(std::atomic<T> &var, T value, std::memory_order mo = std::memory_order_seq_cst)
    \fn template <typename T> QAtomicScopedValueRollback<T>::QAtomicScopedValueRollback(QBasicAtomicInteger<T> &var, T value, std::memory_order mo = std::memory_order_seq_cst)
    \fn template <typename T> QAtomicScopedValueRollback<T>::QAtomicScopedValueRollback(QBasicAtomicPointer<std::remove_pointer_t<T>> &var, T value, std::memory_order mo = std::memory_order_seq_cst)
 
    Assigns \a value to \a var and stores the prior value of \a var internally for
    reverting on destruction.
 
    This is equivalent to:
    \code
    T old_value = var.exchange(new_value, mo);
    // And in the destructor: var.store(old_value, mo);
    \endcode
*/
 
/*!
    \fn template <typename T> QAtomicScopedValueRollback<T>::~QAtomicScopedValueRollback()
 
    Restores the stored value that was current at construction time, or
    at the last call to commit(), to the managed variable.
 
    This is equivalent to:
    \code
    // In the constructor: T old_value = var.load(mo);
    // or: T old_value = exchange(new_value, mo);
    var.store(old_value, mo);
    \endcode
    Where \c{mo} is the same as the one initially passed to the constructor.
    See \l{Memory Order} for the meaning of \c{mo}.
*/
 
/*!
    \fn template <typename T> void QAtomicScopedValueRollback<T>::commit()
 
    Updates the stored value to the managed variable's current value, loaded
    with the same memory order as on construction.
 
    This updated value will be restored on destruction, instead of the original
    prior value.
 
    This is equivalent to:
    \code
    // Given constructor: T old_value = var.load(mo);
    old_value = var.load(mo);  // referesh it
    // And, in the destructor: var.store(old_value, mo);
    \endcode
    Where \c{mo} is the same as the one initially passed to the constructor.
    See \l{Memory Order} for the meaning of \c{mo}.
*/
 
QT_END_NAMESPACE