//-----------------------------------------------------------------------------
// File: a_singleton.h
//
// Desc: An one-instance class
//-----------------------------------------------------------------------------

#ifndef A_SINGLETON_H
#define A_SINGLETON_H

#include <QtGlobal>

// disable the warning regarding 'this' pointers being used in 
// base member initializer list. Singletons rely on this action
// #pragma warning(disable : 4355)

//--------------------------------------------------------------------------------------
// Name: cSingleton
// Desc: A singleton is a type of class of which only one instance may 
//        exist. This is commonly used for management classes used to 
//        control system-wide resources. 
//--------------------------------------------------------------------------------------
template <class T>
class cSingleton
{
public:

    // the singleton must be constructed with a reference to the controlled object
    //--------------------------------------------------------------------------------------
    cSingleton(T& rObject)
    {
        Q_ASSERT_X(!s_pInstance, "constructor", "Only one instance of this class is permitted.");
        s_pInstance = &rObject;
    }

    // the singleton accessor
    //--------------------------------------------------------------------------------------
    ~cSingleton()
    {
        Q_ASSERT_X(s_pInstance, "destructor", "The singleton instance is invalid.");
        s_pInstance = 0;
    }

    // the singleton accessor
    //--------------------------------------------------------------------------------------
    static T& instance()
    {
        Q_ASSERT_X(s_pInstance, "instancing", "The singleton has not yet been created.");
        return (*s_pInstance);
    }

private:

    // Data...
    //--------------------------------------------------------------------------------------
    static T* s_pInstance;

    // Nonexistant Functions...
    //--------------------------------------------------------------------------------------
    cSingleton( const cSingleton& Src);
    cSingleton& operator=( const cSingleton& Src);

};
template <class T> T* cSingleton<T>::s_pInstance = 0;


#endif // A_SINGLETON_H

<...>
#include "a_singleton.h"

// Defines for geting instance
class cMyClass;
#define MY_CLASS_OBJECT cMyClass::instance()

//--------------------------------------------------------------------------------------
// Name: cMyClass
// Desc: Simple example of singleton Qt-class
//--------------------------------------------------------------------------------------
class cMyClass :
        public QObject, public cSingleton<cMyClass>
{
    Q_OBJECT

public:
    // Constructor & destructor
    //----------------------------------------------------------------------------------
    cMyClass(QObject *parent = 0);
<...>

// cMyClass class constructor
//--------------------------------------------------------------------------------------
cMyClass::cMyClass(QObject *parent)
    : QObject(parent), cSingleton<cMyClass>(*this)
{
<...>