Question

#include <iostream>
#include <vector>

using namespace std;

class Parent {
public:
    Parent();
    void method();
};

class Child: public Parent {
public:
    Child();
    void method();
};

int main() {
    vector<Parent> v;
    v.push_back(Parent());
    v.push_back(Child());
    v[0].method();
    v[1].method();
    return 0;
}

Parent::Parent() {}

void Parent::method() {
    cout << "Parent." << endl;
}

Child::Child() {}

void Child::method() {
    cout << "Child." << endl;
}

Basically I’d expect that program to print

Parent.
Child.

but it prints this instead:

Parent.
Parent.

C++ surprises me yet again :).
Why does this happen? What can I do to call Child::method for instances of Child in the vector, and Parent::method for instances of Parent?

I’m using gcc:

gcc version 4.6.1 20110819 (prerelease) (GCC)

like this:

g++ -Wall -Wextra -c main.cpp
g++ -Wall -Wextra -o main main.o

Answer 1

You are encountering the slicing problem.

vector<Parent>.push_back() copies its argument, so it invokes Parent::Parent(const Parent &) (i.e. the copy constructor)^*.

This is really no different to:

Parent x[10];

x[0] = Parent();
x[1] = Child();

_{* Or it might be Parent::operator=(const Parent &), i.e. copy-assignment. I’m tired, and I can’t remember which it needs.}