我正在寻找Singleton Qt实现并找到了this。但我对此有一些疑问。
create
成为QBasicAtomicPointer
的目的是什么? qCallOnce
使用testAndSetRelaxed
有什么意义?获取语义是否已经阻止了之后的任何内存重新排序?qCallOncePerThread
功能的目的是什么? Isn' t qCallOnce
已经是线程安全的吗?我在这里复制建议实施的内容:
call_once.h
#ifndef CALL_ONCE_H
#define CALL_ONCE_H
#include <QtGlobal>
#include <QAtomicInt>
#include <QMutex>
#include <QWaitCondition>
#include <QThreadStorage>
#include <QThread>
namespace CallOnce {
enum ECallOnce {
CO_Request,
CO_InProgress,
CO_Finished
};
Q_GLOBAL_STATIC(QThreadStorage<QAtomicInt*>, once_flag)
}
template <class Function>
inline static void qCallOnce(Function func, QBasicAtomicInt& flag)
{
using namespace CallOnce;
#if QT_VERSION < 0x050000
int protectFlag = flag.fetchAndStoreAcquire(flag);
#elif QT_VERSION >= 0x050000
int protectFlag = flag.fetchAndStoreAcquire(flag.load());
#endif
if (protectFlag == CO_Finished)
return;
if (protectFlag == CO_Request && flag.testAndSetRelaxed(protectFlag,
CO_InProgress)) {
func();
flag.fetchAndStoreRelease(CO_Finished);
}
else {
do {
QThread::yieldCurrentThread();
}
while (!flag.testAndSetAcquire(CO_Finished, CO_Finished));
}
}
template <class Function>
inline static void qCallOncePerThread(Function func)
{
using namespace CallOnce;
if (!once_flag()->hasLocalData()) {
once_flag()->setLocalData(new QAtomicInt(CO_Request));
qCallOnce(func, *once_flag()->localData());
}
}
#endif // CALL_ONCE_H
singleton.h
#ifndef SINGLETON_H
#define SINGLETON_H
#include <QtGlobal>
#include <QScopedPointer>
#include "call_once.h"
template <class T>
class Singleton
{
private:
typedef T* (*CreateInstanceFunction)();
public:
static T* instance(CreateInstanceFunction create);
private:
static void init();
Singleton();
~Singleton();
Q_DISABLE_COPY(Singleton)
static QBasicAtomicPointer<void> create;
static QBasicAtomicInt flag;
static QBasicAtomicPointer<void> tptr;
bool inited;
};
template <class T>
T* Singleton<T>::instance(CreateInstanceFunction create)
{
Singleton::create.store(create);
qCallOnce(init, flag);
return (T*)tptr.load();
}
template <class T>
void Singleton<T>::init()
{
static Singleton singleton;
if (singleton.inited) {
CreateInstanceFunction createFunction = (CreateInstanceFunction)Singleton::create.load();
tptr.store(createFunction());
}
}
template <class T>
Singleton<T>::Singleton() {
inited = true;
};
template <class T>
Singleton<T>::~Singleton() {
T* createdTptr = (T*)tptr.fetchAndStoreOrdered(nullptr);
if (createdTptr) {
delete createdTptr;
}
create.store(nullptr);
}
template<class T> QBasicAtomicPointer<void> Singleton<T>::create = Q_BASIC_ATOMIC_INITIALIZER(nullptr);
template<class T> QBasicAtomicInt Singleton<T>::flag = Q_BASIC_ATOMIC_INITIALIZER(CallOnce::CO_Request);
template<class T> QBasicAtomicPointer<void> Singleton<T>::tptr = Q_BASIC_ATOMIC_INITIALIZER(nullptr);
#endif // SINGLETON_H
如何使用
// myclass.h
#ifndef MYCLASS_H
#define MYCLASS_H
#include <QObject>
class MyClass : public QObject
{
Q_OBJECT
private:
MyClass(QObject* parent = 0);
static MyClass* createInstance();
public:
~MyClass();
static MyClass* instance();
};
#endif // MYCLASS_H
// myclass.cpp
#ifndef MYCLASS_H
#define MYCLASS_H
#include <QObject>
#include "singleton.h"
MyClass::MyClass(QObject* parent):
QObject(parent)
{
}
MyClass* MyClass::createInstance()
{
return new MyClass();
}
MyClass::~MyClass()
{
}
MyClass* MyClass::instance()
{
return Singleton<MyClass>::instance(MyClass::createInstance);
}
#endif // MYCLASS_H
的main.cpp
#include <QTextStream>
#include "myclass.h"
#define MyClassInstance Singleton<MyClass>::instance()
int main(int argc, char* argv[])
{
QTextStream(stdout) << MyClass::instance()->metaObject()->className() << endl;
return 0;
}
答案 0 :(得分:2)
我认为这将足以使用下一个单例实现。我记得,C ++ 11保证静态变量只有一个实例化/初始化。 原始问题是这样的,当多个线程试图同时访问一个实例并且可能出现这种情况时,单例创建两次。
template <typename T, typename D = T>
class Singleton
{
friend D;
static_assert(std::is_base_of_v<T, D>, "T should be a base type for D");
public:
static T& instance();
private:
Singleton() = default;
~Singleton() = default;
Singleton( const Singleton& ) = delete;
Singleton& operator=( const Singleton& ) = delete;
};
template <typename T, typename D>
T& Singleton<T, D>::instance()
{
static D inst;
return inst;
}
// Usage:
class MyClass : public Singleton<MyClass>
{
public:
void foo(){}
};
// Access:
MyClass::instance().foo();
答案 1 :(得分:0)
使用 Singleton 模式的主要概念是将实例化限制为一定数量的对象,通常用于一个。
Q1:原子指针
原子操作不间断地完成,因此处理了多线程实例调用。
Q2:qCallOnce点
此函数检查是否正在执行任何其他线程,如果是,则等待
中的CO_finished标志 do {
QThread::yieldCurrentThread();
}
while (!flag.testAndSetAcquire(CO_Finished, CO_Finished));
Q3:qCallOncePerThread
点 if (!once_flag()->hasLocalData()) {
once_flag()->setLocalData(new QAtomicInt(CO_Request));
我认为每个线程instacnes处理单例类的LocalData