Inheriting Friend Functions
Can friend functions be inherited by derived classes?
Friend functions are not inherited by derived classes. This is because the friend relationship is specific to the class where the friend function is declared. Here's how it works:
Understanding Friend Functions
When you declare a friend function in a class, you're granting it access to the private and protected members of that specific class. This does not extend to derived classes. Each class must explicitly declare its own friends.
Example
Consider a base class Base and a derived class Derived. If Base declares a friend function, this friend function does not automatically become a friend of Derived.
#include <iostream>
class Base {
friend void showData(Base &b, bool derived);
private:
int baseData{10};
};
class Derived : public Base {
private:
int derivedData{20};
// Note: showData is not a friend here.
};
void showData(Base &b, bool showDerived = false) {
std::cout << "\nBase data: " << b.baseData;
if (showDerived) {
std::cout << "\nDerived data: "
<< static_cast<Derived &>(b).derivedData;
}
}
int main() {
Base b;
Derived d;
showData(b, false);
showData(d, true);
}error C2248: 'Derived::derivedData': cannot access private member declared in class 'Derived'In this example, showData() is a friend of Base and can access its private members. However, it is not a friend of Derived, so it cannot access the private derivedData member.
Making a Friend Function for Derived Classes
If you want a friend function to access members of both base and derived classes, you must declare it as a friend in each class:
#include <iostream>
class Base {
friend void showData(Base &b, bool derived);
private:
int baseData{10};
};
class Derived : public Base {
friend void showData(Base &b, bool derived);
private:
int derivedData{20};
};
void showData(Base &b, bool showDerived = false) {
std::cout << "\nBase data: " << b.baseData;
if (showDerived) {
std::cout << "\nDerived data: "
<< static_cast<Derived &>(b).derivedData;
}
}
int main() {
Base b;
Derived d;
showData(b, false);
showData(d, true);
}Base data: 10
Base data: 10
Derived data: 20By declaring showData() as a friend in both Base and Derived, it can now access private members of both classes. This approach keeps the encapsulation intact while granting necessary access where needed.
Friend Classes and Functions
An introduction to the friend keyword, which allows classes to give other objects and functions enhanced access to its members