Thread: Polymorphism in C++

I have been using C++ for a long time now, but this just blows my mind. For my example I made two classes: A and B. Class A was meant to be the interface, where B is a child of A that contains the actual commands. In hope that I could use polymorphism besides the scope of an argument into a function/method, I went ahead and tried shoving and instance B into A.

I was not sure that this would work in the first place which is why I did this test. But that is not the full purpose of this post. What really blew my mind was the output (of the non-pointer version)! It returned 1! But my mind is trying to connivence itself that the outputs should either be an error or to return 5.

The pointer method works just as expected. But I would very much rather use the non-pointer method if at all possible.

Code:

#include <iostream>

using namespace std;

class A
{
public:
	virtual int yay(){return 5;}
};
class B : public A
{
public:
	int yay(){return 1;}
};

int main (int argc, const char * argv[])
{
	A val = B();
	cout << val.yay() << endl << endl;
	A *valp = new B();
	cout << valp->yay() << endl << endl;
	return 0;
}

Thanks for any help!

==Short Version==
1) Whats up with the output?
2) How can I fix it?

Yep, it all has to do with virtual methods. Since your vtable points to B::yay(), you get a "1" when you call valp->yay(). This is just the nature of polymorphism.

If you want to call A's yay(), you can explicitly call it with valp->A::yay().

Originally Posted by jakebird451

I have been using C++ for a long time now, but this just blows my mind. For my example I made two classes: A and B. Class A was meant to be the interface, where B is a child of A that contains the actual commands. In hope that I could use polymorphism besides the scope of an argument into a function/method, I went ahead and tried shoving and instance B into A.

I was not sure that this would work in the first place which is why I did this test. But that is not the full purpose of this post. What really blew my mind was the output (of the non-pointer version)! It returned 1! But my mind is trying to connivence itself that the outputs should either be an error or to return 5.

The pointer method works just as expected. But I would very much rather use the non-pointer method if at all possible.

Code:

#include <iostream>

using namespace std;

class A
{
public:
	virtual int yay(){return 5;}
};
class B : public A
{
public:
	int yay(){return 1;}
};

int main (int argc, const char * argv[])
{
	A val = B();
	cout << val.yay() << endl << endl;
	A *valp = new B();
	cout << valp->yay() << endl << endl;
	return 0;
}

Thanks for any help!

==Short Version==
1) Whats up with the output?
2) How can I fix it?

Your first output can't possibly be '1' - the variable 'val' is of type A..initializing it with an object of type B does nothing to the vtable. It might help to keep in mind that C++ requires that the size of a stack object be known at compile time (you can't simply "squeeze" another type into it at runtime). That sort of polymorphic behavior requires dynamic allocation.

I'm glad you brought up the point of the vtable, but I am till a little unsure why the constructor for B did not throw an error since I tried to construct an object of a different type.