Function overloading with base and derived class references - c++

Question

I have the following classes:
#include <iostream>
using namespace std;
class M
{
};
class N:public M
{
};
class A
{
public:
virtual void f(M& m)=0;
};
class B:public A
{
public:
void f(M& m){cout<<"using M version"<<endl;}
void f(N& n){cout<<"using N version"<<endl;}
};
and the following implementation:
int main()
{
N n;
A &o = *new B();
o.f(n);
//o.f(static_cast<N&>(n));
}
Basically I want "void f(N&)" to be implemented instead of "void f(M&)" when calling "o.f(n)", but not sure how to achieve it.

Answer 1

For the specific behavior that you describe, you can use a static_cast:
f(static_cast<B&>(*pa));
But everybody is right, you're better off using true polymorphism by making the show() method virtual:
#include <iostream>
using namespace std;
class A
{
private:
int a;
public:
A():a(1){}
virtual void show(){cout<<a<<endl;}
};
class B:public A
{
private:
int b;
public:
B():b(2){}
void show(){cout<<b<<endl;}
};
void f(A & a) {a.show();}
int main()
{
A * pa = new A();
f(*pa);
pa = new B();
f(*pa);
}

Answer 2

You should use virtual functions. This is what polymorphism is for: To determine the type of object at runtime. Modify the code as follows by making f() as a virtual function.
#include <iostream>
using namespace std;
class A
{
private:
int a;
public:
A():a(1){}
void show(){cout<<a<<endl;}
virtual void f(A & a) {a.show();}
};
class B:public A
{
private:
int b;
public:
B():b(2){}
void show(){cout<<b<<endl;}
virtual void f(B & a) {a.show();}
};
int main()
{
A * pa = new A();
f(*pa);
pa = new B();
f(*pa);
}

Answer 3

Keep a pointer to B and call f(*pb). Or f(*static_cast<B*>(pa));.
Although I think what you might really want is to make your method virtual.
virtual void show(){cout<<a<<endl;}
It will still call the first f() but will call the show() method of B.
The following example should help you undertand better the different concepts in actions:
#include <iostream>
using namespace std;
class A
{
public:
void show(){cout<<"show: A"<<endl;}
virtual void show_virtual(){cout << "show_virtual: A" << endl; }
};
class B:public A
{
public:
void show(){cout<<"show: B"<<endl;}
virtual void show_virtual(){cout << "show_virtual: B" << endl; }
};
void f(A & a)
{
cout << "f(A&)" << endl;
a.show();
a.show_virtual();
}
void f(B & b)
{
cout << "f(B&)" << endl;
b.show();
b.show_virtual();
}
int main()
{
A * pa = new A();
B * pb = new B();
A * pb_a = pb;
f(*pa);
cout << endl;
f(*pb);
cout << endl;
f(*pb_a);
}
With the following output
f(A&)
show: A
show_virtual: A
f(B&)
show: B
show_virtual: B
f(A&)
show: A
show_virtual: B

Answer 4

Basically I want "void f(N&)" to be implemented instead of "void
f(M&)" when calling "o.f(n)", but not sure how to achieve it.
You can't call "void f(N&)" because there's no such method in class A.
Solution: Declare void f(N&) in class A:
class A
{
public:
virtual void f(M& m)=0;
virtual void f(N& n)=0;
}