Qt 下 Singleton 模式小记

• The Singleton pattern restricts a class so that only one instance can be created. This can be accomplished by making its constructor private or protected and providing an instance() function that creates a new instance if one does not already exist, but returns a pointer or reference to that instance if it does.

Singleton(单件) 模式的意图是：保证类仅有一个实例，并提供一个访问它的全局访问点。

应用于多线程环境的 Singleton 模式又叫做 Double-Checked Locking(双重检查锁定)模式。

代码

• singleton.h

#include <QObject>
#include <QMutex>
#include <QMutexLocker>
class Singleton:public QObject
{
public:
    static Singleton* instance()
    {
        static QMutex mutex;
        if (!m_instance) {
            QMutexLocker locker(&mutex);
            if (!m_instance)
                m_instance = new Singleton;
        }
 
        return m_instance;
    }

private:
    Singleton();
    static Singleton* m_instance;
};

• singleton.cpp

#include <QtCore/QCoreApplication>
#include "singleton.h"

Singleton *Singleton::m_instance = 0;

Singleton::Singleton():
    QObject(qApp)
{
}

在C++中实现单件模式时，何时销毁对象似乎总是一个问题。在Qt中，由于程序中(一般)会有一个QCoreApplication或其派生类的对象，再加上QObject半自动的内存管理方式。我们将QCoreApplication的对象作为该单件对象的父对象，问题就应该迎刃而解了。

QCoreApplication

按照前面的说法：保证类仅有一个实例，并提供一个访问它的全局访问点 。QCoreApplication也应该属于单件模式。不过它的实现有点变态(中性词哈)，看看它的代码：

• qcoreapplication.h

class Q_CORE_EXPORT QCoreApplication : public QObject
{
public:
    QCoreApplication(int &argc, char **argv);
    static QCoreApplication *instance() { return self; }
...
private:
    void init();
    static QCoreApplication *self;
...
};

• qcoreapplication.cpp

QCoreApplication *QCoreApplication::self = 0;
QCoreApplication::QCoreApplication(int &argc, char **argv)
{
    init();
...
}
void QCoreApplication::init()
{
    Q_ASSERT_X(!self, "QCoreApplication", "there should be only one application object");
    QCoreApplication::self = this;
...
}

qApp

有人可能对这个宏感兴趣

• qcoreapplication.h

#define qApp QCoreApplication::instance()

• qapplication.h

#if defined(qApp)
#undef qApp
#endif
#define qApp (static_cast<QApplication *>(QCoreApplication::instance()))

如果我们要派生QCoreApplication或QApplication，可能也会考虑重定义该宏。

参考

• Design Patterns Explained, Chapter 16
• Introduction to Design Patterns in C++ with Qt 4, chapter 16

• http://www.qtcentre.org/wiki/index.php?title=Singleton_pattern