凡科建设网站怎样收录百度,设计服务商,英铭长沙网站建设,企业网络建设文章目录 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 (C) 2016 Intel Corporation.
** Contact: https://www.qt.io/licensing/
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****************************************************************************/#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 Q_QGS_ ## NAME { \typedef TYPE Type; \QBasicAtomicInt guard Q_BASIC_ATOMIC_INITIALIZER(QtGlobalStatic::Uninitialized); \Q_GLOBAL_STATIC_INTERNAL(ARGS) \} } \static QGlobalStaticTYPE, \Q_QGS_ ## NAME::innerFunction, \Q_QGS_ ## 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Q_QGS_ ## 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 Q_QGS_ ## NAME { \typedef TYPE Type; \QBasicAtomicInt guard Q_BASIC_ATOMIC_INITIALIZER(QtGlobalStatic::Uninitialized); \Q_GLOBAL_STATIC_INTERNAL(ARGS) \} } \static QGlobalStaticTYPE, \Q_QGS_ ## NAME::innerFunction, \Q_QGS_ ## 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();
}
