C++ compiler error involving private inheritance

Question

Could someone please explain the following compiler error to me:

struct B
{
};

template <typename T>
struct A : private T
{
};

struct C : public A<B>            
{                                                                             
    C(A<B>);   // ERROR HERE
};

The error at the indicated line is:

test.cpp:2:1: error: 'struct B B::B' is inaccessible
test.cpp:12:7: error: within this context

What exactly is inaccessible, and why?

Answer 1

The problem is name shielding of struct B . Check it out:

struct B{};

struct X{};

template <class T>
struct A : private T
{};

struct C : public A<B>
{
    C(){
          A<X> t1;     // WORKS
 //       A<B> t2;     // WRONG
          A< ::B> t3;  // WORKS
    }   
};

int main () {
}

Answer 2

You are making A privately inherit from B when you do A<B>, and that means that B::B is private so you can't construct a C.

Answer 3

Try A< ::B> or A<struct B>.

Inside of C, unqualified references to B will pick up the so-called injected-class-name, it is brought in through the base class A. Since A inherits privately from B, the injected-class-name follows suit and will also be private, hence be inaccessible to C.

Another day, another language quirk...