Question

In C#, a virtual method of base class can be either overridden or hidden by its derived classes, e.g.

using System;

class A {
 public virtual void M() {
  System.Console.WriteLine("A");
 }
}

class B: A {
 public override void M() {
  System.Console.WriteLine("B");
 }
}

class C: A {
 public new void M() {
  System.Console.WriteLine("C");
 }
}

class P {
 static void Main(string[] args) {
  A b = new B();
  b.M();
  A a = new C();
  a.M();
 }
}

Output:

B
A

How about in C++? Only overridable?

Answer 1

[Update for C++11]

Since C++11, you can use the override keyword to tell the compiler that you’re intending to override the base class virtual function.

void foo() override;

If there is no override keyword but the base class contains a virtual function with the same* signature, then the function is still overriden. Otherwise, hidden.

[Original answer for C++03]

Of course, you can both override and hide a virtual function in C++. You just aren’t very explicit about it (not in C++03 anyway).

example

class A
{
public:
   virtual int f(char);
};

class B : public A
{
public:
    virtual int f(); //hides A::f
};

class C : public A
{
public:
    virtual int f(char); //overrides A::f
};

If the signature coincides with the Base virtual method* then it’s overriding. Otherwise it’s hiding.

So sometimes you accidentally forget a const and end up hiding the function instead of overriding it. The C++11 override attribute is called to solve this problem. If you use this override attribute but the signatures are incompatible, you will get a compiler error.

_{*(or differs only in that the return types are pointers or references to a Base class and a publicly Derived class respectively)}