Class inaccessible after inheritance

Question

If you compile the code below it fails, saying that class B is inaccessible where it is used as an argument to the member function func. Why is this?

Note: if D2 does not inherit from D1 then the error goes away, so somehow the inheritance from D2 makes B inaccessible.

namespace myns {
  class B {};
}

using namespace myns;

class D1 : B {};

class D2 : D1 {
  void func(B b) {}
};

Answer 1

Name lookup finds D2::D1::B , not myns::B . After name lookup, access check is performed, and discovers that D2::D1::B is private.

The namespace is a red herring: the exact same outcome is observed if B is defined in the global namespace.

Answer 2

You need to add the :: operator. Lookup is finding the injected-class-name instead of the myns::B .

class D2 : D1 {
  void func(::B b) {}
};

11.1p5

5 [ Note: In a derived class, the lookup of a base class name will find the injected-class-name instead of the name of the base class in the scope in which it was declared. The injected-class-name might be less accessible than the name of the base class in the scope in which it was declared. —end note ] [Example:

class A { };
class B : private A { };
class C : public B {
  A *p; // error: injected-class-name A is inaccessible
  ::A *q; // OK
};

Answer 3

The default access specifier when inheriting a class/struct is "private", so your problem is this:

class D1 : B {};
class D2 : D1 {};

Add the keyword "public" and voila.

http://ideone.com/T5RpUM

namespace myns {
  class B {};
}

using namespace myns;

class D1 : public B {};

class D2 : public D1 {
  void func(B b) {}
};

int main() {
    D2 d();
}