Instance of C++ template class as a member of another template class

Question

Let's say I have following templated C++ class

#include <cstdint>      

template <uint32_t NO_POINTS>
class A
{
public:
  struct Point
  {
    float x;
    float y;
  };

  A(const Point (&points)[NO_POINTS])
  {
      for (uint32_t point = 0; point < NO_POINTS; point++) {
          table[point] = points[point];
      }
  }
  
private:
  Point table[NO_POINTS];
};

and I would like to use an instance of this class as a private member of the following class:

#include "A.h"

template <uint32_t NO_LUT_POINTS>
class B
{

public:
  B(A<NO_LUT_POINTS>::Point (&table)[NO_LUT_POINTS]) : lut(table){}

private:
  A<NO_LUT_POINTS> lut;
};

#include "B.h"

int main(int argc, char** argv) {

     B<4> foo({{1326.0, 25.0},   {1601.0, 30.0},   {1922.0, 35.0},   {2293.0, 40.0}});
           
    return 0;
}

I have attempted to compile this code but the compiler reports following error A<NO_LUT_POINTS>::Point is not a type . I don't understand what the reason for this error is. Can anybody explain to me why the compiler reports this error?

Answer 1

This is a common mistake with types nested in template classes. You need to add typename to tell the compiler that Point is a type.

...
public:
  B(typename A<NO_LUT_POINTS>::Point const (&table)[NO_LUT_POINTS]) : lut(table){}
...

Beyond solving your problem, however, please notice that Point doesn't depend on the template parameters of A , so you should not nest it in that class. This would remove the necessity for adding typename .