Thread: A simple question about C++

I have 2 programs. Why EX1 is correct but EX 2 is not?

======EX 1=======

Code:

class Base{
public:
void foo() const{ }

};


class Derived: public Base{
public:
int foo() const{ return 1; }
};

======EX 2======

Code:

class Base{
public:
virtual void foo() const{ }

};


class Derived: public Base{
public:
int foo() const{ return 1; }
};

In example 1, there are two foo() methods in the Derived class: the one you created which returns an int, and the one that was inherited from Base. Since the signatures are different, you should be able to compile and call those functions fine.

In example 2, foo() is a virtual method of Base. This means in general that foo() would not be inhereted but overriden explicitly by Derived. Since you changed the signature of foo() in Derived, however, the method isn't necessarily virtual. A v-table can only be constructed if a functions signature remains the same for all derived classes from a base class (which includes those derived classes which rely on the base method).

So that should have made some sense. It should be covered in a chapter of your C++ textbook if you want to go over it in detail.