C++ Friend keyword Friend function
Example
A class or a structure may declare any function it's friend. If a function is a friend of a class, it may access all it's protected and private members:
// Forward declaration of functions.
void friend_function();
void non_friend_function();
class PrivateHolder {
public:
PrivateHolder(int val) : private_value(val) {}
private:
int private_value;
// Declare one of the function as a friend.
friend void friend_function();
};
void non_friend_function() {
PrivateHolder ph(10);
// Compilation error: private_value is private.
std::cout << ph.private_value << std::endl;
}
void friend_function() {
// OK: friends may access private values.
PrivateHolder ph(10);
std::cout << ph.private_value << std::endl;
}
Access modifiers do not alter friend semantics. Public, protected and private declarations of a friend are equivalent.
Friend declarations are not inherited. For example, if we subclass PrivateHolder:
class PrivateHolderDerived : public PrivateHolder {
public:
PrivateHolderDerived(int val) : PrivateHolder(val) {}
private:
int derived_private_value = 0;
};
and try to access it's members, we'll get the following:
void friend_function() {
PrivateHolderDerived pd(20);
// OK.
std::cout << pd.private_value << std::endl;
// Compilation error: derived_private_value is private.
std::cout << pd.derived_private_value << std::endl;
}
Note that PrivateHolderDerived member function cannot access PrivateHolder::private_value, while friend function can do it.
