Local declaration from an inherited struct not recognised

Question

template< int a, int b >
struct aa {
  template< int x, int y >
  struct att { int axx[ a + x ][ b + y ]; };
};   

struct b1 : public aa< 1, 2 > {
  typedef att< 1, 2 > x;
};

template< int q >
struct b2 : public aa< 12, q > {
    typedef att< 1, 2 > x;
};

b1 compiles without a problem (GCC 4.8), but b2 complains

error: 'att' does not name a type
typedef att< 1, 2 > x;
        ^

why??

possible duplicate of [Derived template-class access to base-class member-data](http://stackoverflow.com/questions/1120833/derived-template-class-access-to-base-class-member-data) — juanchopanza, Jan 19 '14 at 14:55
More like `typedef typename aa<12,q>::template att< 1, 2 > x;`. What a mouthful! See [this related question](http://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords). — juanchopanza, Jan 19 '14 at 14:58
@juanchopanza I don't think that's needed as `b1` and `b2` aren't templates.... :) — 0x499602D2, Jan 19 '14 at 14:59
@juanchopanza: make that an answer! But it is an ugly mouthful :) is there a good way to shorten it? My real code has ~ 30 such lines. (yes, it should be 'b2 cmplains') — Wouter van Ooijen, Jan 19 '14 at 15:03
I think all the information is in the second link, which is a better duplicate than the first one. So I will abstain from answering. — juanchopanza, Jan 19 '14 at 15:05

Stuart Golodetz · Answer 1 · 2014-01-19T15:06:46.397

Because non-dependent names such as att<1,2> are not looked up in dependent base classes (like aa<12,q>, which depends on q).

In the case of b1, the base class aa<1,2> is not a dependent base, so everything works fine. For b2, the easiest way is to write typename aa<12,q>::template att<1,2>.

For a relevant reference, see section 9.4.2 (Dependent Base Classes) in C++ Templates: The Complete Guide (Vandevoorde and Josuttis).

Local declaration from an inherited struct not recognised

1 Answers1