Question

I was wondering if it’s possible to override just one function in a class without creating an entirely new class.

I would like bObj1.foo(); to output “foo!” and bObj2.foo() to output “foo?”, but currently they both output “foo!”.

#include <iostream>
using namespace std;

class B {
    public:
    virtual void foo() { cout << "foo!" << endl; }
};

class A {
    public:
    B f();
};

B A::f() {
    B bObj;
    return bObj;
}

class C : public A {
};

int main()
{
    A aObj;
    B bObj1 = aObj.f();
    bObj1.foo(); // "foo!"

    C cObj;
    B bObj2 = cObj.f();
    bObj2.foo(); // "foo?"
}

Answer 1

You can get the behavior that you want with a simple change, which consists in moving the “virtual” behavior to the A and C classes.

Here I modified your application to return the expected result:

#include <iostream>
using namespace std;

class A;

class B {
public:
    B(A& a) : aref(a) {}
    void foo();
private:
    A& aref;
};

class A {
public:
    B f();
    virtual void foo() { cout << "foo!" << endl; }
};

B A::f() {
    B bObj(*this);
    return bObj;
}

class C : public A {
public:
    virtual void foo() { cout << "foo?" << endl; }
};

void B::foo() { aref.foo(); }

int main()
{
    A aObj;
    B bObj1 = aObj.f();
    bObj1.foo(); // "foo!"

    C cObj;
    B bObj2 = cObj.f();
    bObj2.foo(); // "foo?"
}