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.

Methods may declared as friends as well as functions:

class Accesser {
public:
    void private_accesser();
};

class PrivateHolder {
public:
    PrivateHolder(int val) : private_value(val) {}
    friend void Accesser::private_accesser();
private:
    int private_value;
};

void Accesser::private_accesser() {
    PrivateHolder ph(10);
    // OK: this method is declares as friend.
    std::cout << ph.private_value << std::endl;
}

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;
};