嵌套的结构和指针

Question

I am learning about nested structures and came across the following code:

// Stack.h

#ifndef STACK_H
#define STACK_H

struct Stack{
    struct Link{
        void* data;
        Link* next;
        void initialize(void* dat, Link* nxt);
    }* head;
    void initialize();
    void push(void* dat);
    void* peek();
    void* pop();
    void cleanup();
};

#endif // STACK_H

the Link structure is within the scope of Stack and to access Link I would have to use Stack::Link.

I am a bit confused about the pointer head that is declared after the } to close the Link struct.

Does this mean that there is a Link pointer variable named head inside the Stack scope?

What is the effect of defining the head pointer as:

};
Link* head;

vs

}* head; //as per the code above?

Answer 1

There is no difference. Both declarations result in a Stack::head member of type Stack::Link* .

Answer 2

Does this mean that there is a Link pointer variable named head inside the Stack scope?

Yes, that's exactly right.

As to your second question, there is no semantic difference between the two styles of declaration.

Answer 3

Its just a short hand for the semantic. like how we use += . And yes you have a local pointer variable.

struct Stack{
    struct Link{
        void* data;
        Link* next;
        void initialize(void* dat, Link* nxt);
    };

    Link* head;  //Same as code as in your program
    void initialize();
    void push(void* dat);
    void* peek();
    void* pop();
    void cleanup();
};