将类型参数传递给自引用指针

Question

template <class T>
class Node {
    private:
        T m_value;
        //Node* m_ptr;    //(1)
        //Node<T>* m_ptr; //(2)
};

Can someone explain what is the difference between the above two statements (1) and (2)? Both statements seem to compile, but I can't seem to find what the ISO C++ says about them.

Answer 1

They are the same thing, because you declare the pointer inside the template, thus when you create an instance of Node the compiler knows what T is. You don't have to specify the type for a template if it can be deduced, eg from argument types, or in this case from the template instance the pointer belongs to.

template <class T>
class Node {
public:
    T m_value;
    Node* m_ptr;    //(1)
    //Node<T>* m_ptr; //(2)
};

int main()
{
    Node<float> test;
    test.m_ptr = new Node<float>{}; // valid
    test.m_ptr = new Node<bool>{};  // invalid - triggers compiler error
    auto val = test.m_ptr->m_value; // val will be of float type
}

Answer 2

According to the C++ Standard (14.6.1 Locally declared names)

3 The injected-class-name of a class template or class template specialization can be used either as a template-name or a type-name wherever it is in scope. [ Example:

template <class T> struct Base {
Base* p;
};
template <class T> struct Derived: public Base<T> {
typename Derived::Base* p; // meaning Derived::Base<T>
};
template<class T, template<class> class U = T::template Base> struct Third { };
Third<Base<int> > t; // OK: default argument uses injected-class-name as a template

— end example ]

Thus these data member declarations

Node* m_ptr;    //(1)
Node<T>* m_ptr; //(2)

are equivalent because the injected class name Node is used in the scope of the class definition.

Answer 3

They are "the same"

I think that's what it is happening is, in the case of

Node* m_ptr_n

The compiler makes the type match the class type.

In the case of

Node < T>* m_ptr

What it's created is a pointer to a class witch template matches with the "T" type.

If you want to get an int instance that points to a float instance what you can do is pass both types to the template.

template <class T1, class T2, class T3>
    class Node {
    public:
         T1 m_value;
         // Node* m_ptr_n;    //(1)
         Node<T2, T3>* m_ptr; //(2)
    };