QThread call-once semantics?

[brcain]

I'm relatively new to QThread and am changing some code that previously used Boost threads.

Does QThread support the call-once semantics ... similar to Boost code below?

/// Description: Thread-safe singleton class.
 
// Code in header file ...
class cSingleton
{
public:
   static cSingleton * getInstance();
private:
   cSingleton() {}
   static void createInstance();       // This method invoked by boost::call_once
   static boost::once_flag mgOnceFlag; // Special flag used to check if associated
                                       //   boost::call_once routine has been called yet
   static cSingleton * mgInstance;     // Singleton instance
};
 
 
// Code in source file ...
cSingleton * cSingleton::mgInstance(0);
 
// Statically intialized at compile time ... thus not subject to multithreaded
// initialization problems.
boost::once_flag cSingleton::mgOnceFlag(BOOST_ONCE_INIT);
 
cSingleton * cSingleton::getInstance()
{
   // boost::call_once is necesary for thread-safetly because multiple threads
   // can call this method at the same time ... creating two instances of the
   // singleton.  Using boost::call_once(), only one thread is actually able to
   // call createInstance().
   boost::call_once(&cSingleton::createInstance, mgOnceFlag);
   return mgInstance;
}
 
void cSingleton::createInstance()
{
   if(!mgInstance)
      mgInstance = new cSingleton;
}

/// Description: Thread-safe singleton class.

// Code in header file ...
class cSingleton
{
public:
   static cSingleton * getInstance();
private:
   cSingleton() {}
   static void createInstance();       // This method invoked by boost::call_once
   static boost::once_flag mgOnceFlag; // Special flag used to check if associated
                                       //   boost::call_once routine has been called yet
   static cSingleton * mgInstance;     // Singleton instance
};


// Code in source file ...
cSingleton * cSingleton::mgInstance(0);

// Statically intialized at compile time ... thus not subject to multithreaded
// initialization problems.
boost::once_flag cSingleton::mgOnceFlag(BOOST_ONCE_INIT);

cSingleton * cSingleton::getInstance()
{
   // boost::call_once is necesary for thread-safetly because multiple threads
   // can call this method at the same time ... creating two instances of the
   // singleton.  Using boost::call_once(), only one thread is actually able to
   // call createInstance().
   boost::call_once(&cSingleton::createInstance, mgOnceFlag);
   return mgInstance;
}

void cSingleton::createInstance()
{
   if(!mgInstance)
      mgInstance = new cSingleton;
}

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[danadam]

Well, it's about access serialization, so I would use QMutex:

// Singleton.h
#include <QMutex>
 
class Singleton
{
public:
    static Singleton *getInstance();
 
private:
    Singleton();
 
    static Singleton *instance;
    static QMutex instanceMutex;
};
 
// Singleton.cpp
#include <QtDebug>
#include <QMutexLocker>
 
Singleton *Singleton::instance(0);
QMutex Singleton::instanceMutex;
 
Singleton *Singleton::getInstance()
{
    if (!instance) {
        QMutexLocker instanceMutexLocker(&instanceMutex);
        if (!instance) {
            instance = new Singleton();
        }
    }
 
    return instance;
}
 
Singleton::Singleton()
{
    qDebug() << "Singleton ctor()";
}

// Singleton.h
#include <QMutex>

class Singleton
{
public:
    static Singleton *getInstance();

private:
    Singleton();

    static Singleton *instance;
    static QMutex instanceMutex;
};

// Singleton.cpp
#include <QtDebug>
#include <QMutexLocker>

Singleton *Singleton::instance(0);
QMutex Singleton::instanceMutex;

Singleton *Singleton::getInstance()
{
    if (!instance) {
        QMutexLocker instanceMutexLocker(&instanceMutex);
        if (!instance) {
            instance = new Singleton();
        }
    }

    return instance;
}

Singleton::Singleton()
{
    qDebug() << "Singleton ctor()";
}

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[brcain]

Well, it's about access serialization, so I would use QMutex

Yep. I knew I could do that for this case. Maybe call-once semantics is unnecessary with the mutex. I wonder why Boost thought it necessary ... other cases that it might address.

Do you know if Qt has something similar ... just as a mental note?

[danadam]

I wonder why Boost thought it necessary ... other cases that it might address.

Unfortunately I'm not very much familiar with boost, and I cannot think up any reason.

Do you know if Qt has something similar

I've never come across anything similar, but I've been dealing with Qt for only a few months, so it doesn't mean that no such thing exists.

[jacek]

Do you know if Qt has something similar ... just as a mental note?

You can construct it yourself using QMutex or QSemaphore.

[brcain]

You can construct it yourself using QMutex or QSemaphore.

Yep ... did that

Thanks