Cannot call constructor (template-template parameter)

Question

Somehow I can't create an instance of D. I don't know why. The template-template parameter (Allocator_class) seems to be the problem.

#include <vector>

template <template<typename T> class Allocator_class>
class T_b {
public:
    template<typename T> using A = Allocator_class<T>;
};

template< template <typename T> class A>
class C {
public:
    C() { }
};

template<typename Ttb>
class D {
public:
    template<typename T> using A = typename Ttb::template A<T>;
    typedef C<A> Data;

    D(C<A> &data) : _data(data) {}
private:
    Data &_data;
};

int main() {
    typedef T_b<std::allocator> Ttb;    
    C<std::allocator> b;
    D<Ttb>c(b);
}

Error from Clang:

test5.cpp:29:8: error: no matching constructor for initialization of 'D<Ttb>'
      (aka 'D<T_b<std::allocator> >')
        D<Ttb>c(b);
              ^ ~
test5.cpp:16:7: note: candidate constructor (the implicit copy constructor) not viable: no known
      conversion from 'C<std::allocator>' to 'const D<T_b<std::allocator> >' for 1st argument
class D {
      ^
test5.cpp:16:7: note: candidate constructor (the implicit move constructor) not viable: no known
      conversion from 'C<std::allocator>' to 'D<T_b<std::allocator> >' for 1st argument
class D {
      ^
test5.cpp:21:5: note: candidate constructor not viable: no known conversion from 'C<std::allocator>'
      to 'C<A> &' for 1st argument
    D(C<A> &data) : _data(data) {}
    ^

Perhaps a [non-deduced context](http://stackoverflow.com/questions/25245453/what-is-a-nondeduced-context)? — Bo Persson, Feb 08 '17 at 21:52
I think `A` is fine; see [here](http://ideone.com/eljdI9) where I got some code to compile and run with `A`. The difference between my sample and yours is that `C` is templated on a concrete class (not another templated class). Perhaps that's the issue? Too many layers of templating causes the compiler to give up? — R_Kapp, Feb 08 '17 at 21:58
I found that if everything just take the type bundle (`T_b`) as the template parameter, the "thing" can work. This thing is fiendish. — hamster on wheels, Feb 08 '17 at 22:11

score 1 · Accepted Answer · answered Feb 08 '17 at 22:16

1

I can't explain why your code gives an error.

But if you want a solution... using a class specialization and a template template template parameter....

#include <vector>

template <template<typename T> class Allocator_class>
class T_b
 {
   public:
      template<typename T> using A = Allocator_class<T>;
 };

template <template <typename T> class A>
class C
 {
   public:
      C() { }
 };

template <typename>
class D;

template <template <template <typename> class> class X,
          template <typename> class A>
class D<X<A>>
 {
   public:
      using Data = C<A>;

      D (Data & data) : _data(data) {}

   private:
      Data & _data;
 };

int main()
 {
    typedef T_b<std::allocator> Ttb;    

    C<std::allocator> b;

    D<Ttb> c(b);
 }

answered Feb 08 '17 at 22:16

max66

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I am all confused when I look at this code, but I am happy that this can work. Just don't know why ... – hamster on wheels Feb 08 '17 at 22:22
@rxu - it's only another level of templetization; the template argument for `D` is a template type with an argument that isn't a type but a template-template. So template argument of `D` is (if I can use this expression) a template-template-template with a template-template argument. My example (the specialization) show how extract the template-template argument of the argument of `D`. Isn't that clear ? :( – max66 Feb 08 '17 at 22:29

Cannot call constructor (template-template parameter)

1 Answers1