[c++] Extended friend declarations

In C++, the friend keyword allows a function or a class to access the private and protected members of another class. In some cases, you may need to provide access to private or protected members to a large number of functions or classes. Extended friend declarations come in handy in such situations.

What are Extended Friend Declarations?

Extended friend declarations allow an external class or function to access the private or protected members of the declaring class and also its derived classes. This provides a way to grant access to a wider set of classes or functions than just the immediate friend. To use extended friend declarations, the friend keyword is followed by a class name or a function which is already a friend, and a scope resolution operator to specify the class or the function that is allowed access.


Consider a base class Base and a derived class Derived. We want the class Other to access the private members of Base and Derived. Here’s how we can achieve this using extended friend declarations:

class Base {
    int privateData;
    friend class Other; // Extended friend declaration

class Derived : public Base {
    int morePrivateData;
    friend class Other; // Extended friend declaration

class Other {
    void accessPrivateMembers(Base &baseObj) {
        // Access private members of Base using baseObj
        std::cout << baseObj.privateData;

    void accessPrivateMembers(Derived &derivedObj) {
        // Access private members of Derived using derivedObj
        std::cout << derivedObj.privateData << ", " << derivedObj.morePrivateData;

In this example, the Other class is granted access to the private members of both Base and Derived through extended friend declarations.

Advantages of Extended Friend Declarations


Extended friend declarations in C++ allow classes and functions to access the private and protected members of a class and its derived classes. They provide a flexible and convenient way to manage access control in complex scenarios.