Q_GLOBAL_STATIC宏
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
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#include 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 implementation
QT_END_NAMESPACE
#include 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();
}