Re: Error referring to a template within a template - why?

Hi everyone,

I've come up against an issue with template types. Interestingly enough
it seems to trigger a bug in GCC 3.3.4 (it accepts the code but wrongly
replaces the typename), whereas Comeau's online compiler reports it as
an error.

Either way I don't quite understand what the problem is. If someone
could point it out for me, it would be much appreciated!

Comeau gives this error:

"ComeauTest.c", line 37: error: type name is not allowed
  return createValueContainer<R>::DBType<DBT>::createAltValue(pField);
                                         ^

Of course if I take the error literally and remove the type name, it
tells me I need one:

"ComeauTest.c", line 37: error: argument list for class template
 "createValueContainer<R>::DBType [with R=CProject]" is missing
  return createValueContainer<R>::DBType::createAltValue(pField);
                                  ^

Whereas GCC accepts the code, but treats R == DBT, even though they're
not the same type. The compilable code is below.

Any suggestions as to where I'm going wrong?

Thanks,
Adam.

#include <string>

// Cut down datatypes to make this sample easier to read
class IValue {};
template <class R, typename T>
class TField {};

struct CProject {
  struct Fields {
    static TField<CProject, int> Test;
  };
};
TField<CProject, int> CProject::Fields::Test;

template <class R>
struct createValueContainer {

  template <typename DBT>
  struct DBType {
    template <typename T>
    inline static IValue *createAltValue(const TField<R, T>& pField)
      throw ()
    {
      // Do something with a variable of type DBT
      return NULL;
    }
  };

};

template <typename DBT>
struct createAltValueContainer {
  template <class R, typename T>
  inline static IValue *createAltValue(const TField<R, T>& pField)
    throw ()
  {
    // This next line doesn't seem to work. GCC gives an error and
    // reports that R = std::string when it should be CProject.
    return createValueContainer<R>::DBType<DBT>::createAltValue(pField);

  }
};

int main(void)
{
  createAltValueContainer<std::string>::createAltValue(
    CProject::Fields::Test);
}