No class template named X in templated class

Question

When tryin to compile this (CRTP-like) code with GCC 4.6.0:

template<template<class> class T> struct A;

template<class T> 
struct B: A<B<T>::template X> {
    template <class U> struct X { U mem; };
};

B<int> a;

I get the errormessage "test.cpp:3:26: error: no class template named ‘X’ in ‘struct B<int>’". Why does X seem to be invisible outside the class definition?

My compiler has a problem with this: : A< B::template X> The X is an undeclared identifier. I'm frankly confused by your syntax, what's B::template supposed to be? Besides I've never seen: template class T. If anything, I think I'm going to learn something from you; can you please explain the importance of the syntax, what these do? — leetNightshade, Apr 26 '11 at 18:01
I'm not sure, but maybe Johannes's answer from this topic might help here : http://stackoverflow.com/questions/4420828/another-bug-in-g-clang-c-templates-are-fun — Nawaz, Apr 26 '11 at 18:04
@leetNightshade: I'm not sure but I think you're error message is actually saying the same thing as mine: that X is not visible outside B (this is how I understand it anyway). About your second question: "template class T" means that T is templated itself. Like that you can use something like T inside the class definition of A. — Tom De Caluwé, Apr 26 '11 at 18:07

score 4 · Accepted Answer · edited May 23 '17 at 11:47

As Emile Cormier correctly points out here the problem is that at the place of instantiation of A, B is still an incomplete type, and you cannot use the inner template.

The solution for that is moving the template X outside of the template B. If it is independent of the particular instantiation T of the template B, just move it to the namespace level, if it is dependent on the instantiation, you can use type traits:

template <typename T>
struct inner_template 
{
   template <typename U> class tmpl { U mem; }; // can specialize for particular T's
};
template <typename T>
struct B : A< inner_template<T>::template tmpl >
{
};

Emile Cormier · Answer 2 · 2011-04-26T18:09:00.597

2

struct B is still considered an incomplete type when you specify A<B<T>::template X> as the base class.

edited Apr 26 '11 at 18:09

answered Apr 26 '11 at 18:03

Emile Cormier

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score 1 · Answer 3 · answered Apr 26 '11 at 18:08

1

You're trying to use a member of B as a parent of B creating a recursive-esque situation. For example this doesn't compile either:

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

struct B : public A<B::nested>
{
        struct nested {};
};

answered Apr 26 '11 at 18:08

Mark B

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That's not true: I'm passing B as a template parameter, I'm not inheriting from it. – Tom De Caluwé Apr 26 '11 at 18:13
@Tom De Caluwé I changed my answer to show that even as a template parameter it's still treated as an incomplete type which can't be used as a template parameter. – Mark B Apr 26 '11 at 18:18

No class template named X in templated class

3 Answers3