C++ passing template parameter pack from one variadic template to another causes compiler error

Question

The following code causes compiler errors, but only for gcc (up to 5.2) and clang (up to 3.7). VS2015 compiles without promblems. (For g++ you have to use the option -std=c++14)

//************************************************
template <int... items>
struct Sequence {
//************************************************
public:
  enum { size = sizeof...(items) };
  operator int() const { return data[rank - 1]; }
  Sequence& operator=(const Sequence& s) { rank = s.rank; return *this; }
  int get_rank() const { return rank; }
  bool first() { rank = 1; return true; }
  bool last() { rank = size; return true; }
  bool next() { return (rank < size) ? ++rank, true : false; }
  bool prev() { return (rank > 1) ? --rank, true : false; }
  bool is_first() { return rank == 1; }
  bool is_last() { return rank == size; }
protected:
  static const int data[sizeof... (items)];
  int rank = 1;
};

template <int... items> 
const int Sequence<items...>::data[sizeof...(items)] = { items... };


//************************************************
template <unsigned N, template <unsigned> class F, int... items> 
struct generate_sequence {
//************************************************
  typedef typename generate_sequence<N - 1, F, F<N>::value, items...>::result result;
};

//************************************************
template <template <unsigned> class F, int... items> 
struct generate_sequence<0, F, items...> {
//************************************************
  typedef Sequence<F<0>::value, items...> result;
};


//************************************************
template <int... coeffs>
struct polynomial {
//************************************************
  template <int var, int a0, int... ai> struct ipoly { enum { value = a0 + var * ipoly<var, ai...>::value }; };
  template <int var, int a0> struct ipoly<var, a0> { enum { value = a0 + 0 * var }; };

  template <unsigned index>
  class number_generator { public: enum { value = ipoly<index + 1, coeffs...>::value }; };
};


//************************************************
template <unsigned N>
class NaturalNumbers : public generate_sequence<N - 1, polynomial<0,1>::number_generator>::result {};
//************************************************

//************************************************
template <unsigned N, int... coeffs>
class PolynomialNumbers : public generate_sequence<N - 1, polynomial<coeffs...>::number_generator>::result {};
//************************************************

int main() {
  NaturalNumbers<10> nn;
  PolynomialNumbers<10,0,1> pn;
}

The compiler output is as follows:

bug.cpp:59:98: error: type/value mismatch at argument 2 in template parameter list for 'template<unsigned int N, template<unsigned int <anonymous> > class F, int ...items> struct generate_sequence'
 class PolynomialNumbers : public generate_sequence<N - 1, polynomial<coeffs...>::number_generator>::result {};
                                                                                                  ^
bug.cpp:59:98: note:   expected a class template, got 'polynomial<coeffs ...>::number_generator'
bug.cpp:59:101: error: expected '{' before 'result'
 class PolynomialNumbers : public generate_sequence<N - 1, polynomial<coeffs...>::number_generator>::result {};
                                                                                                     ^
bug.cpp: In function 'int main()':
bug.cpp:64:27: error: non-template type 'PolynomialNumbers' used as a template
   PolynomialNumbers<10,0,1> pn;
                           ^

Is this a compiler bug or is the code somehow wrong? For me the key lies in the line

expected a class template, got 'polynomial::number_generator'

of the compiler output. The compiler obviously does not realize that 'polynomial::number_generator' in fact is a template. What do you mean?

Barry · Accepted Answer · 2015-08-27T21:37:41.367

Since number_generator is a dependent name, you have to prefix it with the template keyword:

template <unsigned N, int... coeffs>
class PolynomialNumbers 
: public generate_sequence<
      N - 1, 
      polynomial<coeffs...>::template number_generator
                          // ^^^^^^^^
      >::result 
{};

As a side-note, it might be better to avoid passing template templates anywhere and try to rewrite your number_generator to be more like a metafunction class:

class number_generator {
public: 
    template <unsigned index>
    using apply = ipoly<index + 1, coeffs...>;
};

That'll make all your code easier since types are first-class citizens and anything that isn't a type (i.e. a template template or a value) is much more difficult to deal with.

C++ passing template parameter pack from one variadic template to another causes compiler error

1 Answers1