qglobalstatic.h

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// Copyright (C) 2021 Intel Corporation.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
#ifndef QGLOBALSTATIC_H
#define QGLOBALSTATIC_H
 
#include <QtCore/qatomic.h>
#include <QtCore/qtclasshelpermacros.h>
 
#include <atomic>           // for bootstrapped (no thread) builds
#include <type_traits>
 
QT_BEGIN_NAMESPACE
 
namespace QtGlobalStatic {
enum GuardValues {
    Destroyed = -2,
    Initialized = -1,
    Uninitialized = 0,
    Initializing = 1
};
 
template <typename QGS> union Holder
{
    using Type = typename QGS::QGS_Type;
    using PlainType = std::remove_cv_t<Type>;
 
    static constexpr bool ConstructionIsNoexcept = noexcept(QGS::innerFunction(nullptr));
    Q_CONSTINIT static inline QBasicAtomicInteger<qint8> guard = { QtGlobalStatic::Uninitialized };
 
    // union's sole member
    PlainType storage;
 
    Holder() noexcept(ConstructionIsNoexcept)
    {
        QGS::innerFunction(pointer());
        guard.storeRelaxed(QtGlobalStatic::Initialized);
    }
 
    ~Holder()
    {
        pointer()->~PlainType();
        std::atomic_thread_fence(std::memory_order_acquire); // avoid mixing stores to guard and *pointer()
        guard.storeRelaxed(QtGlobalStatic::Destroyed);
    }
 
    PlainType *pointer() noexcept
    {
        return &storage;
    }
 
    Q_DISABLE_COPY_MOVE(Holder)
};
}
 
template <typename Holder> struct QGlobalStatic
{
    using Type = typename Holder::Type;
 
    bool isDestroyed() const noexcept { return guardValue() <= QtGlobalStatic::Destroyed; }
    bool exists() const noexcept { return guardValue() == QtGlobalStatic::Initialized; }
    operator Type *()
    {
        if (isDestroyed())
            return nullptr;
        return instance();
    }
    Type *operator()()
    {
        if (isDestroyed())
            return nullptr;
        return instance();
    }
    Type *operator->()
    {
        Q_ASSERT_X(!isDestroyed(), "Q_GLOBAL_STATIC",
                   "The global static was used after being destroyed");
        return instance();
    }
    Type &operator*()
    {
        Q_ASSERT_X(!isDestroyed(), "Q_GLOBAL_STATIC",
                   "The global static was used after being destroyed");
        return *instance();
    }
 
protected:
    static Type *instance() noexcept(Holder::ConstructionIsNoexcept)
    {
        static Holder holder;
        return holder.pointer();
    }
    static QtGlobalStatic::GuardValues guardValue() noexcept
    {
        return QtGlobalStatic::GuardValues(Holder::guard.loadAcquire());
    }
};
 
#define Q_GLOBAL_STATIC_WITH_ARGS(TYPE, NAME, ARGS)                         \
    QT_WARNING_PUSH                                                         \
    QT_WARNING_DISABLE_CLANG("-Wunevaluated-expression")                    \
    namespace { struct Q_QGS_ ## NAME {                                     \
        typedef TYPE QGS_Type;                                              \
        static void innerFunction(void *pointer)                            \
            noexcept(noexcept(std::remove_cv_t<QGS_Type> ARGS))             \
        {                                                                   \
            new (pointer) QGS_Type ARGS;                                    \
        }                                                                   \
    }; }                                                                    \
    Q_CONSTINIT static QGlobalStatic<QtGlobalStatic::Holder<Q_QGS_ ## NAME>> NAME; \
    QT_WARNING_POP
    
 
#define Q_GLOBAL_STATIC(TYPE, NAME, ...)                                    \
    Q_GLOBAL_STATIC_WITH_ARGS(TYPE, NAME, (__VA_ARGS__))
 
QT_END_NAMESPACE
#endif // QGLOBALSTATIC_H