Qt Singleton实施 [英] Qt Singleton implementation
问题描述
我一直在寻找Singleton Qt实现,并发现了此.但我对此有疑问.
I was looking for a Singleton Qt implementation and found this. but I have some question about it.
- 将
create
设为QBasicAtomicPointer
的目的是什么? - 如果以前使用过fetchAndStoreAcquire,在
qCallOnce
中使用testAndSetRelaxed
有什么意义?习得语义不是已经在阻止它之后的任何内存重新排序了吗? -
qCallOncePerThread
功能的作用是什么?qCallOnce
已经不是线程安全的吗?
- What is the purpose of making
create
aQBasicAtomicPointer
? - What is the point in
qCallOnce
of usingtestAndSetRelaxed
if previously we have used fetchAndStoreAcquire ? Isn't the acquire semantic already preventing any memory reordering after it ? - What is the purpose of the
qCallOncePerThread
function ? Isn'tqCallOnce
already thread-safe ?
我在此处复制建议的实施内容:
I copy the contents of the suggested implementation here:
call_once.h
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
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
使用方法
// 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
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;
}
推荐答案
我认为使用下一个单例实现就足够了.我记得,C ++ 11保证静态变量只有一个实例化/初始化. 最初的问题是,如果有多个线程尝试同时访问一个实例,并且有可能会发生两次创建单例的情况.
I think that is will be enough to use next singleton implementation. As I remember, C++11 gurantees that there will be only one instancing/initialization for a static variable. Original problem was in case, when more than one thread tries to access an instance in same time and there were possible a situation, when singleton was created twice.
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();
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