calling a virtual function through a reference: derived class' override gets called

Question

I have the following code:

#include <iostream>
using namespace std;

class Parent {
public:
    virtual void f() { cout << "Parent" << endl; }
};

class Child : public Parent {
public:
    void f() { cout << "Child" << endl; }
};

void t1(Parent * p) { p->f(); }
void t2(Parent & p) { p.f(); }

int main() {
    Child a;

    t1(&a);
    t2(a);

    return 0;
}

I tested this both in Visual Studio 2013 and ideone.com, and both got the result:

Child
Child

I can understand t1 calling Child::f() as the result of dynamic binding, but the second one has me puzzled - I expected the Parent & to "fix" the type, so t2 would call Parent::f(). Am I misunderstanding the rules? How does one explain this behavior?

`I expected the Parent & to "fix" the type` What is this expectation based on? Can you point to a piece of documentation that misled you so? — Igor Tandetnik, May 03 '15 at 18:24
I know it works on pointers; so references also follow the same rules? — martin_ljchan, May 03 '15 at 18:24
Yes. Though it looks like you don't completely understand those rules; can we help you? — edmz, May 03 '15 at 18:26
@IgorTandetnik: it's most likely based on lectures that I didn't completely stay awake in ... : ) I think I need to reread the docs on this — martin_ljchan, May 03 '15 at 18:28
@martin_ljchan And skip parties at night before attending _important lectures_ on the next morning, next time! — πάντα ῥεῖ, May 03 '15 at 18:32

score 3 · Accepted Answer · edited May 23 '17 at 12:06

3

This is what polymorphism does.

I expected the Parent & to "fix" the type, so t2 would call Parent::f().

Well, you expected wrong.

Am I misunderstanding the rules?

Yes.

How does one explain this behavior?

By picking up a book on C++ and reading the chapter on polymorphism.
Your Parent& works just as a Parent* would: by allowing virtual dispatch.

The important thing to note is that pointers (and references) do not invoke polymorphism. So wherever you read that only pointers invoke polymorphism was doubly wrong. Polymorphism is invoked by object access. It just so happens that, due to the slicing problem, it is impossible to invoke virtual dispatch except through a pointer or reference.

Further reading

edited May 23 '17 at 12:06

Community

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answered May 03 '15 at 18:28

Lightness Races in Orbit

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On [invoking](https://stackoverflow.com/questions/15188894/why-doesnt-polymorphism-work-without-pointers-references) virtual dispatch – edmz May 03 '15 at 18:30
I must have somehow missed the part about virtual dispatch works for pointers _and references_. Thanks everyone for the help; the extra reading is also appreciated :) – martin_ljchan May 03 '15 at 18:42

calling a virtual function through a reference: derived class' override gets called

1 Answers1