Question

I am not very clear about this code

outer is a class and the inner a struct, can anyone help me explain it?

class Stack {
    struct Link {
        void* data;
        Link* next;
        Link(void* dat, Link* nxt):
        data(dat),next(nxt) {}
    }* head;
public:
    Stack():head(0) {}
    ~Stack() {
        require(head==0,"Stack not empty");
    }
    void push(void* dat) {
        head = new Link( dat, head );
    }
    void peek() const {
        return head ? head->data : 0;
    }

    void* pop() {
        if(head == 0)  return 0;
        void* result = head->data;
        Link* oldHead = head;
        head = head->next;
        delete oldHead;
        return result;
    }
};

my question is focus on the first few lines

class Stack {
    struct Link {
        void* data;
        Link* next;
        Link(void* dat, Link* nxt):
        data(dat),next(nxt) {}
    }* head;

what the relation between class Stack and struct Link?

Answer 1

Link is declared inside Stack, and since it’s private by default, it can’t be used outside the class.

Also, Stack has a member head which is of type Link*.

The only difference between class and struct is the default access level – public for struct and private for class, so don’t let “struct declared inside a class” confuse you. Other than the access level, it’s the same as “class declared inside a class” or “struct declared inside a struct”.