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?

Unrelated to the problem, What does C(A) mean here? – Aman Aggarwal Feb 10 '12 at 05:36 — Aman Aggarwal, Feb 10 '12 at 05:36
@fizzbuzz: Constructor taking an `A` by value. – Xeo Feb 10 '12 at 05:37 — Xeo, Feb 10 '12 at 05:37
@Xeo, oh yes, A is just a type. – Aman Aggarwal Feb 10 '12 at 05:49 — Aman Aggarwal, Feb 10 '12 at 05:49

Xeo · Accepted Answer · 2012-02-10T06:39:36.880

6

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...

edited Feb 10 '12 at 06:39

answered Feb 10 '12 at 05:35

Xeo

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Good question. My answer looked like "Try `A< ::B>`. Why? Beats me..." at first. Now, the error message refers to `B::B`, the constructor, so that was about the only option. And no, I don't know why it tries to select a member function inside a template argument list... – Xeo Feb 10 '12 at 05:41
So, is this parsing quirk intentional, a compiler bug (unlikely since both GCC and Comeau give this error), or under/mis-specification of parsing in the standard? – HighCommander4 Feb 10 '12 at 05:50
1

All three MSVC, GCC and Clang spew this error, so it must be something in the spec.. I just asked on the llvm IRC channel, will report back. – Xeo Feb 10 '12 at 05:52
2

I also opened up a [follow-up question](http://stackoverflow.com/q/9223441/500104). – Xeo Feb 10 '12 at 06:04

Dmitriy Kachko · Answer 2 · 2012-02-10T05:49:26.390

4

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 () {
}

edited Feb 10 '12 at 05:49

answered Feb 10 '12 at 05:44

Dmitriy Kachko

2,804
1
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score -1 · Answer 3 · answered Feb 10 '12 at 05:32

-1

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.

answered Feb 10 '12 at 05:32

Seth Carnegie

73,875
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1

Wrong, `A` has to construct the `B` base subobject, and it's accessible there. – Xeo Feb 10 '12 at 05:33
What is `B::B`? B's implicitly declared default constructor? – HighCommander4 Feb 10 '12 at 05:33
@HighCommander `B::B` is the implicitly declared default constructor that is made `private` in `A`. – Seth Carnegie Feb 10 '12 at 05:34

C++ compiler error involving private inheritance

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