Returning class pointer, when asked for struct pointer (C++)

Question

I have a skeleton of a project I need to implement a Doubly Linked List (no using stl) and the way the class is implemented is to inherit all is methods from a struct like so:

struct IDoubleList { 
virtual IDoubleNode * getHead() = 0;
virtual IDoubleNode * getTail() = 0;
virtual void setHead(IDoubleNode * head) = 0;
virtual void setTail(IDoubleNode * tail) = 0;
virtual void addBack(int value) = 0;
};

class DoubleList : public IDoubleList {
public:
virtual IDoubleNode * getHead();
virtual IDoubleNode * getTail();
virtual void addBack(int value);
virtual void setHead(IDoubleNode * head);
virtual void setTail(IDoubleNode * tail);
private:
DoubleNode* m_Head;
DoubleNode* m_Tail;
};

As you can getters and setters use the struct, not the class to return/pass pointers. My question is how can I use the methods in he object m_Tail is pointing to. I tried using m_Tail.setNext(newNode); where setNext is method in the DoubleNode class but that says the expression must have a class type.

Also when I return/pass a DoubleNode* should I be casting or something to IDoubleNode*? or maybe its the other way around?

PS been a while since ive used C/C++, maybe I'm forgetting something about function pointers? idk so lost right now

Thanks in advanced, let me know if you need any more info

There is no such thing as a distinction between class pointers and struct pointers. (That wouldn't seem to answer your question, but it should be commented to the headline). — leftaroundabout
– leftaroundabout, Commented Jan 31, 2012 at 23:44
Why have you defined an interface for which there is a single implementation? The question is a bit rethorical, if there is only one implementation, creating an interface (pure abstract class) will add complexity and offer no benefit. Reconsider your design. — David Rodríguez - dribeas
– David Rodríguez - dribeas, Commented Feb 1, 2012 at 0:39
read my post, i have the skeleton i can only implement classes given to me nothing else. — Dreken105
– Dreken105, Commented Feb 1, 2012 at 1:39

Seth Carnegie · Accepted Answer · 2012-01-31 23:46:04Z

1

First of all, it should be m_Tail->setNext(newNode), not m_Tail.setNext(newNode) because m_Tail is a pointer. You could also do (*m_Tail).setNext(newNode), the point is that you have to dereference the pointer somehow.

No, you do not need to cast a pointer from a DoubleList to IDoubleList when passing it to a function expecting an IDoubleList. This is because every DoubleList is a IDoubleList, so no casting is required. Also, the only difference between a struct and a class is the default access level of the members (public for a struct, private for a class).

answered Jan 31, 2012 at 23:46

Seth Carnegie

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3 Comments

Dreken105 Over a year ago

thanks a lot, i knew there was definitely a way to use a dot operator. for some reason using -> was giving me a error:expression must have pointer-to-class type

Mahesh Over a year ago

@AnkurVashi -> operator can only be with pointers pointing to an object. For example, foo obj;, obj is not pointer to an object of type foo but the object itself. So, you have to use . operator to access its members.

Seth Carnegie Over a year ago

I think you were doing something wrong with your syntax somewhere; if (*a).b() works, then a->b() should also work.

Dietmar Kühl · Accepted Answer · 2012-01-31 23:51:30Z

First, you should make IDoubleNode a base class of your DoubleNode (if you haven't already). Then you need to dynamic_cast<DoubleNode*>(ptr) in the methods taking a IDoubleNode* to get hold of your DoubleNode (and e.g. throw an exception if this fails). Also, make sure you use pointer notation when accessing members of pointers, i.e. ptr->member instead of ptr.member: the latter notation only works for objects. You could use (*ptr).member but this is unnecessarily contrived.

Finally, don't forget to tell your instructor to set reasonable assignments next time because this is an entirely pointless exercise: this is bad example for using object oriented approaches (you can subvert type-safety via the base class interface) and it is pointless to use object oriented approaches in a data structures class.

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