Question

I implemeted a template singleton class along with static member definition:

template <typename T> class SingletonT
{
public:
    static T *GetInstance()
    {
        if (!s_instance)
        {
            s_instance = new T;
        }
        return s_instance;
    }
private:
    static T *s_instance;
};
template <typename T> T *SingletonT<T>::s_instance = 0;

It compiles fine and works fine.
Now, I’m trying to create a singleton that will use custom function instead of operator new:

template <typename T, T *(*f)()> class PolymorphSingletonT
{
public:
    static T *GetInstance()
    {
        if (!s_instance)
        {
            s_instance = f();
        }
        return s_instance;
    }
private:
    static T *s_instance;
};
template <typename T, T *(*f)()> T *PolymorphSingletonT<T, T *(*f)()>::s_instance = 0;

I get a compile error on the last line saying error C2146: syntax error : missing ')' before identifier 'f'

I use MSVC2005 compiler.

Update
Now I ended up with functor approach:

template <typename T, typename F> class PolymorphSingletonT
{
public:
    static T *GetInstance()
    {
        if (!s_instance)
        {
            s_instance = F::Create();
        }
        return s_instance;
    }
private:
    static T *s_instance;
};
template <typename T, typename F> T *PolymorphSingletonT<T, F>::s_instance = 0;

However, it forces me to wrap my creation function with a functor. It only adds a couple of code lines and works perfectly, but anyway I’m still interested in getting it without functors.

Answer 1

The last line should be:

template <typename T, T *(*f)()> T *PolymorphSingletonT<T,f>::s_instance = 0;

Don’t repeat the type signature, use the names you’ve specified.