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override identifier in C++

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  • Difficulty Level : Medium
  • Last Updated : 13 Jun, 2022
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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 same 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. 

The programmer’s intentions can be made clear to the compiler by override. If the override identifier is used with a member function, the compiler makes sure that the member function exists in the base class, and also the compiler restricts the program to compile otherwise.

Let’s understand through the following example: 

CPP




// 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;
}

Output

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++.

CPP




// 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)

prog.cpp:17:7: error: 'void derived::func(int)'
marked 'override', but does not override
  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.

This article is contributed by MAZHAR IMAM KHAN. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.


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