What is an undefined reference ?

Posted on 2005-03-03
Medium Priority
Last Modified: 2010-04-01

I'm working on a program where I have several classes inheriting from a sort of mother-class.

One is a binary search tree, and another one is a stack. I have a function (a member function of the search tree) that uses the stack . . which uses nodes from the search tree. . . . like so :

class Tree {}
class Stack : public Tree {}

void afunction
        // s is a stack, p is a pointer to a  node in the tree
         . . .

I have an error "  [Linker error] undefined reference to `Stack::push(node*)'  "

My question is this . . what's a linker error ? For that, what is an undefined reference ? ( Especially in relation to this particular error . . )

Question by:princess_of_reason
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LVL 30

Expert Comment

ID: 13452269
Hi princess_of_reason,
> >I have an error "  [Linker error] undefined reference to `Stack::push(node*)'  "

Your class does not have a push member function.

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Expert Comment

ID: 13452280
From what you've posted, the Stack class doe not have a push member function, so you can't call s.push

If you need that functionallity, you need to add the push method to your class.
LVL 30

Expert Comment

ID: 13452304
> >what's a linker error ? For that, what is an undefined reference ? (
> >Especially in relation to this particular error . . )

You usally get this type of linker error when you have declared a method, but you have not defined it.

class foo
  int ImNotImplemented();
  int ImImplemented()
     return 0;

foo myfoo;
myfoo.ImNotImplemented();//Will cause linker error
myfoo.ImImplemented();//This is OK
myfoo.NoSuchFunction();//Will cause compile error

If you declare a method, but don't have implementation code, that will usually cause a linker error.
If you don't declare it at all, then you'll get a compile error.

You need both implementation and declaration.

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Author Comment

ID: 13453822
Err .. altogether my code ended up being really long and messy, so I posted "   . . . .  "  instead. My stack does have the push function, and half a dozen other things. . . lol.

I'm sorry for the confusion . . . I'm just trying to figure out what the error means.

So I guess somehow it's not finding push ? Hmm . .
LVL 30

Accepted Solution

Axter earned 2000 total points
ID: 13453845
>>So I guess somehow it's not finding push ? Hmm . .

You may have a push method, but it may not have the correct argument type that the linker is looking for.

Since p is a pointer to a node, your push method needs to take that exact type of argument, and it needs to be implemented.

If you post your class declaration, and the implementation for push, we could point exactly where the problems at.

Author Comment

ID: 13469445
Thank you, that answers my question. :)

I'd rather figure out what's wrong exactly myself, as this program is a homework assignment. I just needed to know what those terms meant. :)

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