What does the virtual keyword mean when overriding a method?

Question

What does the keyword virtual do when overriding a method? I\'m not using it and everything works fine.

Does every compiler behave the same in this

Answer 1

You cannot override a member function without it.

You can only hide one.

struct Base {
   void foo() {}
};

struct Derived : Base {
   void foo() {}
};

Derived::foo does not override Base::foo; it simply hides it because it has the same name, such that the following:

Derived d;
d.foo();

invokes Derived::foo.

virtual enables polymorphism such that you actually override functions:

struct Base {
   virtual void foo() {}
};

struct Derived : Base {
   virtual void foo() {} // * second `virtual` is optional, but clearest
};

Derived d;
Base& b = d;
b.foo();

This invokes Derived::foo, because this now overrides Base::foo — your object is polymorphic.

(You also have to use references or pointers for this, due to the slicing problem.)

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• Derived::foo doesn't need to repeat the virtual keyword because Base::foo has already used it. This is guaranteed by the standard, and you can rely on it. However, some think it best to keep that in for clarity.