override identifier in C++

Function overriding is a redefinition of the base class function in its derived class with the same signature i.e. return type and parameters. 
But there may be situations when a programmer makes a mistake while overriding that function. So, to keep track of such an error, C++11 has come up with the override identifier.

If the compiler comes across this identifier, it understands that this is an overridden version of the function of the base class. It will make the compiler check the base class to see if there is a virtual function with this exact signature. And if there is not, the compiler will show an error.

Let's understand through the following example: 

// A CPP program without override keyword, here
// programmer makes a mistake and it is not caught
#include <iostream>
using namespace std;

class Base {
public:
    // user wants to override this in
    // the derived class
    virtual void func() { cout << "I am in base" << endl; }
};

class derived : public Base {
public:
    // did a silly mistake by putting
    // an argument "int a"
    void func(int a)
    {
        cout << "I am in derived class" << endl;
    }
};

// Driver code
int main()
{
    Base b;
    derived d;
    cout << "Compiled successfully" << endl;
    return 0;
}

Compiled successfully

Explanation: Here, the user intended to override the function func() in the derived class but did a silly mistake and redefined the function with a different signature. Which was not detected by the compiler. However, the program is not actually what the user wanted. So, to get rid of such silly mistakes to be on the safe side, the override identifier can be used. 

Below is a C++ example to show the use of override identifier in C++.

// A CPP program that uses override keyword so
// that any difference in function signature is
// caught during compilation
#include <iostream>
using namespace std;

class Base
{
  public:
    // user wants to override this in
    // the derived class
    virtual void func()
    {
        cout << "I am in base" << endl;
    }
};

class derived : public Base
{
  public:
    // did a silly mistake by putting
    // an argument "int a"
    void func(int a) override
    {
        cout << "I am in derived class" << endl;
    }
};

int main()
{
    Base b;
    derived d;
    cout << "Compiled successfully" << endl;
    return 0;
}

Output(Error)

main.cpp:23:10: error: ‘void derived::func(int)’ marked ‘override’, but does not override
   23 |     void func(int a) override
      |          ^~~~

In short, it serves the following functions. It helps to check if: 

• There is a method with the same name in the parent class.
• The method in the parent class is declared as "virtual" which means it was intended to be rewritten.
• The method in the parent class has the same signature as the method in the subclass.