Why does C++ not allow inherited friendship?

Question

Because I may write Foo and its friend Bar (thus there is a trust relationship).

But do I trust the people who write classes that are derived from Bar?
Not really. So they should not inherit friendship.

Any change in the internal representation of a class will require a modification to anything that is dependent on that representation. Thus all members of a class and also all friends of the class will require modification.

Therefore if the internal representation of Foo is modified then Bar must also be modified (because friendship tightly binds Bar to Foo). If friendship was inherited then all class derived from Bar would also be tightly bound to Foo and thus require modification if Foo's internal representation is changed. But I have no knowledge of derived types (nor should I. They may even be developed by different companies etc). Thus I would be unable to change Foo as doing so would introduce breaking changes into the code base (as I could not modify all class derived from Bar).

Thus if friendship was inherited you are inadvertently introducing a restriction on the ability to modify a class. This is undesirable as you basically render useless the concept of a public API.

Note: A child of Bar can access Foo by using Bar, just make the method in Bar protected. Then the child of Bar can access a Foo by calling through its parent class.

Is this what you want?

class A
{
    int x;
    friend class B;
};

class B
{
    protected:
       // Now children of B can access foo
       void foo(A& a, int n) { a.x = n; }
};

class D : public B
{
    public:
        foo(A& a, int n)
        {
            B::foo(a, n + 5);
        }
};

Why is friendship not at least optionally inheritable in C++?

I think that the answer to your first question is in this question: "Do your father's friends have access to your privates?"

A friended class may expose its friend through accessor functions, and then grant access through those.

class stingy {
    int pennies;
    friend class hot_girl;
};

class hot_girl {
public:
    stingy *bf;

    int &get_cash( stingy &x = *bf ) { return x.pennies; }
};

class moocher {
public: // moocher can access stingy's pennies despite not being a friend
    int &get_cash( hot_girl &x ) { return x.get_cash(); }
};

This allows finer control than optional transitivity. For example, get_cash may be protected or may enforce a protocol of runtime-limited access.

C++ Standard, section 11.4/8

Friendship is neither inherited nor transitive.

If friendship would be inherited, then a class that wasn't meant to be a friend would suddenly have access to your class internals and that violates encapsulation.

Because it's just unnecessary.

The usage of the friend keyword is itself suspicious. In term of coupling it's the worst relationship (way ahead of inheritance and composition).

Any change to the internals of a class have a risk to impact the friends of this class... do you really want an unknown number of friends ? You would not even be able to list them if those who inherit from them could be friends also, and you would run in the risk of breaking your clients code each time, surely this is not desirable.

I freely admit that for homework/pet projects dependency is often a far away consideration. On small size projects it doesn't matter. But as soon as several persons work on the same project and this grows into the dozens of thousands of lines you need to limit the impact of changes.

This bring a very simple rule:

Changing the internals of a class should only affect the class itself

Of course, you'll probably affect its friends, but there are two cases here:

• friend free function: probably more of a member function anyway (I am think std::ostream& operator<<(...) here, which is not a member purely by accident of the language rules
• friend class ? you don't need friend classes on real classes.

I would recommend the use of the simple method:

class Example;

class ExampleKey { friend class Example; ExampleKey(); };

class Restricted
{
public:
  void forExampleOnly(int,int,ExampleKey const&);
};

This simple Key pattern allows you to declare a friend (in a way) without actually giving it access to your internals, thus isolating it from changes. Furthermore it allows this friend to lend its key to trustees (like children) if required.