A implemetion of Singleton Pattern

• Download Singleton_demo.zip - 3.5 KB

Introduction

When you implement your application system, you may find some objects that should have only one instance throughout the application lifetime. You can implement it using global variable, but it is difficult to manage many global variables in application. Singleton Pattern is a good choice. the aim of the pattern is to ensure a class only has one instance, and provide a global point of access to it.In this article I will describe a simple singleton implementations

How is it working

template <class TInstance>
class CSingletonModel 
{
// forbid to new CSingletonModel directly
protected:
CSingletonModel()
{
    printf("CSingletonModel");
};
virtual ~CSingletonModel() 
{ 
    printf("~CSingletonModel"); 
};
// forbid to copy construct function
private:
CSingletonModel(const CSingletonModel &copy);
CSingletonModel & operator = (const CSingletonModel &copy);



protected:
//the only instance of class
static TInstance * m_pInstance;
protected:
//initialize instance
virtual BOOL InitInstance(){ return TRUE; }
//the event function before the GetInstance() return
virtual BOOL AfterGetInstance() { return TRUE; }
public:
//get the only instance
static TInstance * GetInstance()
{
//create the instance
    if (m_pInstance == NULL)
    {    
        m_pInstance = new TInstance; 
//init object.
        if (!m_pInstance->InitInstance())
        {
            delete m_pInstance;
            m_pInstance = NULL;
            return (m_pInstance);
        }
    } 
    m_pInstance->AfterGetInstance();
    return (m_pInstance);
}
//destroy the only Instance
static void DestroyInstance()
{
    if (m_pInstance)
    {
        delete m_pInstance;
        m_pInstance = NULL;
    }
};

template <class TInstance>
TInstance* CSingletonModel<TInstance>::m_pInstance = NULL;

Using the code

class CTest:public CSingletonModel<CTest>
{
public:
 CTest()
 {
  printf("CTest");
 };
 ~CTest()
 {
  printf("~CTest");
 };
public:
 virtual BOOL InitInstance()
 {
  return TRUE;
 }
 virtual BOOL AfterGetInstance()
 {
  return TRUE;
 }
};
int _tmain(int argc, _TCHAR* argv[])
{
//the first method
 CTest *pTest1 = CTest::GetInstance();
 if(pTest1)
 {
  printf("%s","get the only instance of class 1");
 }
 CTest::DestroyInstance();
//the second method
 CTest *pTest2 = CSingletonModel<CTest>::GetInstance();
 if(pTest2)
 {
  printf("%s","get the only instance of class 2");
 }
 CSingletonModel<CTest>::DestroyInstance();
 return 0;
}