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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

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****************************************************************************/#include <QtCore/qglobal.h>#ifndef QGLOBALSTATIC_H
#define QGLOBALSTATIC_H#include <QtCore/qatomic.h>QT_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. C++11 also requires this behavior.
// Clang and Intel CC masquerade as GCC when compiling on Linux.
//
// Apple's libc++abi 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 don't 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_guard<QBasicMutex> 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 QGlobalStatic<TYPE,                                              \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 QGlobalStatic<TYPE,                                              \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 don't 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_guard<QBasicMutex> 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C++11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();   
}