Singleton pattern in C++

Question

I have a question about the singleton pattern.

I saw two cases concerning the static member in the singleton class.

First it is an object, like this

Answer 1

A better approach is to create a singleton class. This also avoids the instance availability check in the GetInstance() function. This can be achieved using a function pointer.

class TSingleton;

typedef TSingleton* (*FuncPtr) (void);

class TSingleton {

TSingleton(); //prevent public object creation
TSingleton  (const TSingleton& pObject); // prevent copying object
static TSingleton* vObject; // single object of a class

static TSingleton* CreateInstance   (void);
static TSingleton* Instance     (void);
public:

static FuncPtr  GetInstance; 
};


FuncPtr TSingleton::GetInstance = CreateInstance;
TSingleton* TSingleton::vObject;

TSingleton::TSingleton()
{
}

TSingleton::TSingleton(const TSingleton& pObject)
{
}

TSingleton* TSingleton::CreateInstance(void)
{
if(vObject == NULL){

    // Introduce here some code for taking lock for thread safe creation
    //...
    //...
    //...

    if(vObject == NULL){

        vObject = new TSingleton();
        GetInstance = Instance;
    }
}

return vObject;
}

TSingleton* TSingleton::Instance(void)
{

return vObject;

}

void main()
{

TSingleton::GetInstance(); // this will call TSingleton::Createinstance()

TSingleton::GetInstance(); // this will call TSingleton::Instance()

// all further calls to TSingleton::GetInstance will call TSingleton::Instance() which simply returns already created object. 

}

Answer 2

I agree with Billy. In 2nd approach we are dynamically allocating memory from the heap using new. This memory remains always and never gets freed, unless a call to delete is been made. Hence the Global pointer approach creates a memory leak.

class singleton
{
    private:
        static singleton* single;
        singleton()
        {  }
        singleton(const singleton& obj)
        {  }

    public:
        static singleton* getInstance();
        ~singleton()
        {
            if(single != NULL)
            {
                single = NULL;
            }
        }
};

singleton* singleton :: single=NULL;
singleton* singleton :: getInstance()
{
    if(single == NULL)
    {
        single = new singleton;
    }
    return single;
}

int main() {
    singleton *ptrobj = singleton::getInstance();
    delete ptrobj;

    singleton::getInstance();
    delete singleton::getInstance();
    return 0;
}