C++ Friend keyword Friend class
Example
A whole class may be declared as friend. Friend class declaration means that any member of the friend may access private and protected members of the declaring class:
class Accesser {
public:
void private_accesser1();
void private_accesser2();
};
class PrivateHolder {
public:
PrivateHolder(int val) : private_value(val) {}
friend class Accesser;
private:
int private_value;
};
void Accesser::private_accesser1() {
PrivateHolder ph(10);
// OK.
std::cout << ph.private_value << std::endl;
}
void Accesser::private_accesser2() {
PrivateHolder ph(10);
// OK.
std::cout << ph.private_value + 1 << std::endl;
}
Friend class declaration is not reflexive. If classes need private access in both directions, both of them need friend declarations.
class Accesser {
public:
void private_accesser1();
void private_accesser2();
private:
int private_value = 0;
};
class PrivateHolder {
public:
PrivateHolder(int val) : private_value(val) {}
// Accesser is a friend of PrivateHolder
friend class Accesser;
void reverse_accesse() {
// but PrivateHolder cannot access Accesser's members.
Accesser a;
std::cout << a.private_value;
}
private:
int private_value;
};
