STL iterator of superclass calling subclass method

Question

I have not used C++ in a while, so I'm not sure what the right way is to do what I'm trying to do. I have the following classes:

class A {
public:
   virtual string printStuff() { return "A"; };
};

class B : public A {
public:
   string printStuff() { return "B"; };
};

class C : public A {
public:
   string printStuff() { return "C"; };
};

I have an STL vector:

vector<A> vec;

It contains a lot of class A and B objects. When I do:

for (vector<A>::iterator iter = vec.begin(); iter != vec.end(); ++iter) {
   iter->printStuff();
}

All it prints is "A". I would like it to use class B and C's methods.

Thank you.

`vector` can only possibly hold objects of type `A`. Look up ["object slicing"](http://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c). — juanchopanza, Jul 23 '14 at 19:17
Your vector holds a bunch of object sliced to A, polymorphism is lost (store unique_ptr's) — , Jul 23 '14 at 19:19

score 4 · Accepted Answer · answered Jul 23 '14 at 19:17

4

A std::vector<A> can only contain objects of type A. If you insert an object of a derived class type, it will be sliced. If you want polymorphic behaviour, use std::vector<A*> (or, better, std::vector<std::unique_ptr<A>>).

answered Jul 23 '14 at 19:17

Brian Bi

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Thanks. What is the advantage of using unique_ptr over ? – Squall Leohart Jul 23 '14 at 19:21
1

@SquallLeohart there is no possibility of leaking memory or other resources with a smart pointer – Brian Bi Jul 23 '14 at 19:22
It seems like my C++ does not support unique_ptr. Any workaround? – Squall Leohart Jul 23 '14 at 19:29
@SquallLeohart If you can't upgrade your compiler, I guess you'll just have to use `A*`. – Brian Bi Jul 23 '14 at 19:32
@SquallLeohart: What compiler are you using, and did you `#include `? – Mooing Duck Jul 23 '14 at 19:47
c++ , and yes I did include memory – Squall Leohart Jul 23 '14 at 19:51

STL iterator of superclass calling subclass method

1 Answers1