C++ inheritance, calling the given classes function instead of its parent? - c++

Question

Really bad title, couldn't think of how to word it, sorry.
So say I had the following code:
class A {
virtual int getSize() {
return 0;
}
}
class B : public A {
int getSize() {
return 32;
}
}
void doStuff(A a) {
std::cout << a.getSize() << std::endl;
}
int main() {
B b;
doStuff(b);
}
It would print out 0, however I want it to print out 32. In other words, I want to pass it the class and it prints out that classes function, so I could create a class C, where the size is 64, and if I pass that C instance to the doStuff function, I want it to print 64.
Is there any way I can do this in C++, would I have to use templates or some fancy C++ feature I don't know about?

Answer 1

A one-byte patch:
void doStuff(A &a) {
std::cout << a.getSize() << std::endl;
}
Your version takes the argument by value, which means that the function makes a copy of b (a copy which is an A) and then calls the copy's getSize(). In this version, the function takes the argument by reference, and calls b's own getSize(), which is B::getSize().

Answer 2

You should use pointers, or even better: smart pointers! That way, the function of the runtime type gets called. It's a basic example of polymorhpism. If you want to avoid pointers, Beta's slicing approach is equally valid.
#include <iostream>
#include <memory>
class A {
virtual int getSize() {
return 0;
}
}
class B : public A {
virtual int getSize() {
return 32;
}
}
void doStuff(std::shared_ptr<A> a) {
std::cout << a->getSize() << std::endl;
}
int main() {
std::shared_ptr<A> b(new B());
doStuff(b); // Will output '32'.
}
This should correctly call the function as implemented by B.

Answer 3

Slicing the object is one approach, and in addition I think you're asking for, I think, a pretty straightforward use of polymorphism in C++. http://www.cplusplus.com/doc/tutorial/polymorphism/
That's almost immediately applicable, just call your class A Shape, and B and C could be Square and Triangle. Your DoStuff function could take a pointer to a Shape, then you can pass it a triangle or a square, and when you deference the Shape in the function, it will call the correct function.
So you'd have (also you need to make the members public, I think):
class A {
public:
virtual int getSize() {
return 0;
}
};
class B : public A {
public:
int getSize() {
return 32;
}
};
void doStuff(A* a) {
std::cout << a->getSize() << std::endl;
}
int main() {
B b;
doStuff(&b);
}