C++: Virtual methods

Question

I have the following piece of code (#includes and using namespace std omitted):

class A {
    public:
        void call(){callme();}
    private:
        virtual void callme() {cout << "I'm A" << endl;}
};

class B : public A {
    private:
        virtual void callme() {cout << "I'm B" << endl;}
};

class C : public B {
    public:
        virtual void callme(){ cout << "I'm C" << endl;}
};

int main(){
    vector<A> stuff = {
        A(), B(), C(),
    };
    stuff[0].call(); // output: I'm A
    stuff[1].call(); // output: I'm A
    stuff[2].call(); // output: I'm A
    return 0;
}

As stated in the comments, the output of the above program is:

I'm A
I'm A
I'm A

However, I would like that C++ automatically recognizes the type with which the corresponding element was created. I.e. I would like that C++ outputs

I'm A
I'm B
I'm C

(That is, the compiler shall pick the proper subclass for me.)

Is this possible in this scenario (i.e. if all the elements come out of a vector)?

See: [What is the slicing problem?](http://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c) — jrok, Jun 25 '13 at 14:08

sasha.sochka · Accepted Answer · 2013-06-25T14:20:26.837

2

Member functions virtuality works only when you call them from pointer to the actual object, not from an object itself, because in your example objects were automatically statically upcasted to class A. Change your code to:

std::vector<std::unique_ptr<A>> stuff = {
    std::unique_ptr<A>(new A()), 
    std::unique_ptr<A>(new B()), 
    std::unique_ptr<A>(new C()),
};
stuff[0]->call(); 
stuff[1]->call(); 
stuff[2]->call();

edited Jun 25 '13 at 14:20

answered Jun 25 '13 at 14:09

sasha.sochka

14,395
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1

`s/A */std::unique_ptr/`... – Griwes Jun 25 '13 at 14:10
OP not neccessarily knows about smart pointers, especially if he asks such a basic question – sasha.sochka Jun 25 '13 at 14:11
4

Then it's the right time to teach him about them. BEFORE teaching him owning raw pointers, FFS. – Griwes Jun 25 '13 at 14:12
Then it's even more important to not even start with owning raw pointers. – Xeo Jun 25 '13 at 14:12
@sasha.sochka all the more reason to use a safe alternative to raw pointers – djf Jun 25 '13 at 14:12
@sasha.sochka I agree with Griwes. Either what he suggested, or `boost::ptr_vector` – Bartek Banachewicz Jun 25 '13 at 14:12
1

OK, guys, fixed. That's true the previous version was a horrible thing to be in production code, but that was an example. I'll make a lesson not to show such examples anymore – sasha.sochka Jun 25 '13 at 14:19

score 1 · Answer 2 · answered Jun 25 '13 at 14:37

For C++ Polymorphism, you should use either pointer or reference. You could do like this

int main(){
         vector<A*> stuff;
         stuff.push_back(new A);
         stuff.push_back(new B);
         stuff.push_back(new C);
         stuff[0]->call(); // output: I'm A
         stuff[1]->call(); // output: I'm A
         stuff[2]->call(); // output: I'm A
         while (!stuff.empty()){
                 delete stuff.back();
                 stuff.pop_back();
         }
         return 0;
 }

Reference: http://www.cplusplus.com/doc/tutorial/polymorphism/

Warning! Look at comments to my answer. You also have non-safe code — sasha.sochka, Jun 25 '13 at 15:01

C++: Virtual methods

2 Answers2