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番禺网站建设制作,塑模毕业设计代做网站,云主机 免费,功能性的网站文章目录 目的Q_GLOBAL_STATIC源代码分析涉及到原子操作 以及静态变量初始化顺序代码实现 目的 由Q_GLOBAL_STATIC宏#xff0c; 引发的基于线程安全的Qt 单例模式的使用。 Q_GLOBAL_STATIC /************************************************************************… 文章目录 目的Q_GLOBAL_STATIC源代码分析涉及到原子操作 以及静态变量初始化顺序代码实现 目的 由Q_GLOBAL_STATIC宏 引发的基于线程安全的Qt 单例模式的使用。 Q_GLOBAL_STATIC /************************************************************************* ** ** Copyright © 2016 Intel Corporation. ** Contact: https://www.qt.io/licensing/ ** ** This file is part of the QtCore module of the Qt Toolkit. ** ** \(QT_BEGIN_LICENSE:LGPL\) ** Commercial License Usage ** Licensees holding valid commercial Qt licenses may use this file in ** accordance with the commercial license agreement provided with the ** Software or, alternatively, in accordance with the terms contained in ** a written agreement between you and The Qt Company. For licensing terms ** and conditions see https://www.qt.io/terms-conditions. For further ** information use the contact form at https://www.qt.io/contact-us. ** ** GNU Lesser General Public License Usage ** Alternatively, this file may be used under the terms of the GNU Lesser ** General Public License version 3 as published by the Free Software ** Foundation and appearing in the file LICENSE.LGPL3 included in the ** packaging of this file. Please review the following information to ** ensure the GNU Lesser General Public License version 3 requirements ** will be met: https://www.gnu.org/licenses/lgpl-3.0.html. ** ** GNU General Public License Usage ** Alternatively, this file may be used under the terms of the GNU ** General Public License version 2.0 or (at your option) the GNU General ** Public license version 3 or any later version approved by the KDE Free ** Qt Foundation. The licenses are as published by the Free Software ** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3 ** included in the packaging of this file. Please review the following ** information to ensure the GNU General Public License requirements will ** be met: https://www.gnu.org/licenses/gpl-2.0.html and ** https://www.gnu.org/licenses/gpl-3.0.html. ** ** \(QT_END_LICENSE\) ** ****************************************************************************/#include QtCore/qglobal.h#ifndef QGLOBALSTATIC_H #define QGLOBALSTATIC_H#include QtCore/qatomic.hQT_BEGIN_NAMESPACEnamespace QtGlobalStatic { enum GuardValues {Destroyed -2,Initialized -1,Uninitialized 0,Initializing 1 }; }#if !QT_CONFIG(thread) || defined(Q_COMPILER_THREADSAFE_STATICS) // some compilers support thread-safe statics // The IA-64 C ABI requires this, so we know that all GCC versions since 3.4 // support it. C11 also requires this behavior. // Clang and Intel CC masquerade as GCC when compiling on Linux. // // Apples libcabi however uses a global lock for initializing local statics, // which will block other threads also trying to initialize a local static // until the constructor returns … // We better avoid these kind of problems by using our own locked implementation.#if defined(Q_OS_UNIX) defined(Q_CC_INTEL) // Work around Intel issue ID 6000058488: // local statics inside an inline function inside an anonymous namespace are global // symbols (this affects the IA-64 C ABI, so OS X and Linux only)

define Q_GLOBAL_STATIC_INTERNAL_DECORATION Q_DECL_HIDDEN

#else

define Q_GLOBAL_STATIC_INTERNAL_DECORATION Q_DECL_HIDDEN inline

#endif#define Q_GLOBAL_STATIC_INTERNAL(ARGS) \Q_GLOBAL_STATIC_INTERNAL_DECORATION Type *innerFunction() { \struct HolderBase { ~HolderBase() noexcept { if (guard.loadRelaxed() QtGlobalStatic::Initialized) \guard.storeRelaxed(QtGlobalStatic::Destroyed); } }; \static struct Holder : public HolderBase { \Type value; \Holder() \noexcept(noexcept(Type ARGS)) : value ARGS { guard.storeRelaxed(QtGlobalStatic::Initialized); } } holder; \return holder.value; } #else // We dont know if this compiler supports thread-safe global statics // so use our own locked implementationQT_END_NAMESPACE #include QtCore/qmutex.h #include mutex QT_BEGIN_NAMESPACE#define Q_GLOBAL_STATIC_INTERNAL(ARGS) \Q_DECL_HIDDEN inline Type *innerFunction() { \static Type *d; \static QBasicMutex mutex; \int x guard.loadAcquire(); \if (Q_UNLIKELY(x QtGlobalStatic::Uninitialized)) { \const std::lock_guardQBasicMutex locker(mutex); \if (guard.loadRelaxed() QtGlobalStatic::Uninitialized) { \d new Type ARGS; \static struct Cleanup { ~Cleanup() { \delete d; \guard.storeRelaxed(QtGlobalStatic::Destroyed); } } cleanup; \guard.storeRelease(QtGlobalStatic::Initialized); } } \return d; } #endif// this class must be POD, unless the compiler supports thread-safe statics template typename T, T *(innerFunction)(), QBasicAtomicInt guard struct QGlobalStatic {typedef T Type;bool isDestroyed() const { return guard.loadRelaxed() QtGlobalStatic::Destroyed; }bool exists() const { return guard.loadRelaxed() QtGlobalStatic::Initialized; }operator Type *() { if (isDestroyed()) return nullptr; return innerFunction(); }Type *operator()() { if (isDestroyed()) return nullptr; return innerFunction(); }Type operator-(){Q_ASSERT_X(!isDestroyed(), Q_GLOBAL_STATIC, The global static was used after being destroyed);return innerFunction();}Type operator(){Q_ASSERT_X(!isDestroyed(), Q_GLOBAL_STATIC, The global static was used after being destroyed);return *innerFunction();} };#define Q_GLOBAL_STATIC_WITH_ARGS(TYPE, NAME, ARGS) \namespace { namespace QQGS ## NAME { \typedef TYPE Type; \QBasicAtomicInt guard Q_BASIC_ATOMIC_INITIALIZER(QtGlobalStatic::Uninitialized); \Q_GLOBAL_STATIC_INTERNAL(ARGS) } } \static QGlobalStaticTYPE, \QQGS ## NAME::innerFunction, \QQGS ## NAME::guard NAME;#define Q_GLOBAL_STATIC(TYPE, NAME) \Q_GLOBAL_STATIC_WITH_ARGS(TYPE, NAME, ())QT_END_NAMESPACE #endif // QGLOBALSTATIC_H 源代码分析 Q_GLOBAL_STATIC 用于定义全局静态变量 #define Q_GLOBAL_STATIC(TYPE, NAME) \Q_GLOBAL_STATIC_WITH_ARGS(TYPE, NAME, ())Q_GLOBAL_STATIC_WITH_ARGS 带有参数的全局静态变量宏 在匿名命名空间内定义了命名空间QQGS ## NAME该命名空间内定义了 类型typedef TYPE Type原子操作变量QBasicAtomicInt guard 初始化QtGlobalStatic::Uninitialized内联函数Type *innerFunction() 通过类模板QGlobalStatic来定义全局静态变量 NAME #define Q_GLOBAL_STATIC_WITH_ARGS(TYPE, NAME, ARGS) \namespace { namespace QQGS ## NAME { \typedef TYPE Type; \QBasicAtomicInt guard Q_BASIC_ATOMIC_INITIALIZER(QtGlobalStatic::Uninitialized); \Q_GLOBAL_STATIC_INTERNAL(ARGS) } } \static QGlobalStaticTYPE, \QQGS ## NAME::innerFunction, \QQGS ## NAME::guard NAME; 内联函数innerFunction enum GuardValues {Destroyed -2,Initialized -1,Uninitialized 0,Initializing 1 };#define Q_GLOBAL_STATIC_INTERNAL(ARGS) \Q_GLOBAL_STATIC_INTERNAL_DECORATION Type *innerFunction() { \struct HolderBase { ~HolderBase() noexcept { if (guard.loadRelaxed() QtGlobalStatic::Initialized) \guard.storeRelaxed(QtGlobalStatic::Destroyed); } }; \static struct Holder : public HolderBase { \Type value; \Holder() \noexcept(noexcept(Type ARGS)) : value ARGS { guard.storeRelaxed(QtGlobalStatic::Initialized); } } holder; \return holder.value; } #else // We dont know if this compiler supports thread-safe global statics // so use our own locked implementationQT_END_NAMESPACE #include QtCore/qmutex.h #include mutex QT_BEGIN_NAMESPACE 采用双重检查方式来初始化全局静态变量 #define Q_GLOBAL_STATIC_INTERNAL(ARGS) \Q_DECL_HIDDEN inline Type *innerFunction() { \static Type d; \static QBasicMutex mutex; \int x guard.loadAcquire(); \if (Q_UNLIKELY(x QtGlobalStatic::Uninitialized)) { \const std::lock_guardQBasicMutex locker(mutex); \if (guard.loadRelaxed() QtGlobalStatic::Uninitialized) { \d new Type ARGS; \static struct Cleanup { ~Cleanup() { \delete d; \guard.storeRelaxed(QtGlobalStatic::Destroyed); } } cleanup; \guard.storeRelease(QtGlobalStatic::Initialized); } } \return d; } #endif涉及到原子操作 以及静态变量初始化顺序 atomic atomic_load与atomic_store Load是C11标准的介绍 用于原子访问全局和静态变量的新加载和存储操作。 load操作可以保证多线程环境下全局变量和静态变量的原子加载 避免数据竞争和线程不安全问题 静态变量初始化顺序 全局变量初始化在主函数之前静态局部变量初始化在第一次调用这个静态局部变量时 代码实现 mainwindow.h static MainWindow getInstance();mainwindow.cpp Q_GLOBAL_STATIC(MainWindow,mainwindow)static MainWindow* MainWindow::getInstance(){return mainwindow();
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