MSVC：STL容器中的“使用类模板需要模板参数列表”

Question

I have an issue with nested template classes on MSVC 12 and GCC 4.8. I've reduced it to the following snippet:

#include <vector>
#include <list>

template <typename A, typename B>
struct Base
{
  template <typename A2, typename B2> struct InBase { };
};

template <typename A, typename B>
struct Derived : public Base<A, B>
{
  typedef Base<A, B> MyBase;

  // this works on GCC 4.8
  typedef typename MyBase::template InBase<A, B> MyInBase;
  // this works on MSVC 12
  typedef MyBase::InBase<A, B> MyInBase;

  typedef std::vector<MyInBase*> MyInBaseVector;
  typedef std::list<MyInBase*> MyInBaseList;

  MyInBaseList list;
};

When I use the MSVC variant of MyInBase on GCC, it just tells me to add typename and template , which I find understandable. When I use the GCC variant on MSVC (which I believe is the correct one), it reports this:

deptypes.cpp(20) : error C2955: 'Base<A,B>::InBase' : use of class template requires template argument list
    deptypes.cpp(7) : see declaration of 'Base<A,B>::InBase'
    deptypes.cpp(24) : see reference to class template instantiation 'Derived<A,B>' being compiled
C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\vector(648) : error C2955: 'Base<A,B>::InBase' : use of class template requires template argument list
    deptypes.cpp(7) : see declaration of 'Base<A,B>::InBase'
    deptypes.cpp(7) : see declaration of 'Base<A,B>::InBase'
deptypes.cpp(21) : error C2955: 'Base<A,B>::InBase' : use of class template requires template argument list
    deptypes.cpp(7) : see declaration of 'Base<A,B>::InBase'
C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\list(859) : error C2955: 'Base<A,B>::InBase' : use of class template requires template argument list
    deptypes.cpp(7) : see declaration of 'Base<A,B>::InBase'
    deptypes.cpp(7) : see declaration of 'Base<A,B>::InBase'

Is the MSVC error correct? If yes, how can I fix it?

Answer 1

MSVC is rejecting valid code here, as N3337 14.2 (temp.names) p4 states:

When the name of a member template specialization appears after . or -> in a postfix-expression or after a nested-name-specifier in a qualified-id , and the object expression of the postfix-expression is type-dependent or the nested-name-specifier in the qualified-id refers to a dependent type, but the name is not a member of the current instantiation (14.6.2.1), the member template name must be prefixed by the keyword template . Otherwise the name is assumed to name a non-template.

Given this, I checked your sample against ICC 13 and Clang 3.3 on Godbolt : ICC correctly rejects this MSVC-ism with nontype "Base<A, B>::InBase [with A=A, B=B]" is not a template while accepting the conformant ( typename / template qualified) version, whereas Clang, interestingly enough, accepts both the conformant and the nonconformant typedefs! My local copy of MSVC 11 (VS2012) also rejects the version with template and typename in it, as well as passing a template argument list as the error suggests (which is ill-formed in any case).

It appears that the template keyword is what is tripping MSVC up, too, which prevents GCC 4.9's suggestion of using an otherwise-redundant Derived::MyBase::template InBase<A, B> to force the expression into a dependent context without using typename from working.

The Visual Studio devs are already aware of a very similar issue to this . As to working around it, though? You'll just have to use a #ifdef _MSC_VER block to cage the MSVC-specific version of the typedef.

For pedants: ICC, for all it gets ripped on in some corners of the world, does conform to the C++ standards in practice, as it uses the same EDG frontend as Comeau C++.