Question

I have a template class with private constructor and destructor:

template <typename T>
class Client
{
    private:
        Client(...) {}
        ~Client()   {}

        template <typename U>
        friend class Client<T>& initialize(...);
};


template <typename T>
Client<T> initialize(...) 
{
      Client<T> client = new Client<T>(...);
}

I’m not sure of the correct syntax for the friend. Can anybody help?

Answer 1

Ignoring the ellipses (which I assume mean “a bunch of parameters that aren’t relevant to the question”), this should work:

template <typename T>
class Client
{
private:
    Client() {} // Note private constructor
public:
    ~Client() {} // Note public destructor

    // Note friend template function syntax
    template<typename U>
    friend Client<U> initialize();
};


template<typename T>
Client<T> initialize() 
{
      /* ... */
}

Notice that the friend declaration is essentially the same as the function declaration but with the friend keyword prefixed before the return type.

Also, the destructor is public so that users of initialize() will be able to destruct the returned instance of Client<>. The constructor is still private so only initialize() can create instances of it.

The above code should allow this to work:

int main()
{
    Client<int> client = initialize<int>();
}