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?
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.
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