Compilation error when typedef an std::array

Question

I define a type called inputTy using std::array (c++11) , the dimension of the array declared as an external constant integer d.

namespace project {
  namespace types{
    extern const int d;
    typedef std::array<double, d> inputTy;
  }
}

Why do I get such compilation error?

../../include/types.h:16:29: error: the value of ‘d’ is not usable in a constant expression
 typedef std::array<double, d> inputTy;
                             ^
../../include/types.h:15:18: note: ‘d’ was not initialized with a constant expression
 extern const int d;
              ^

Thanks for your help.

I think the value of `d` must be compile-time constant. When it is extern, how should the compiler know? — helb, May 13 '15 at 13:39
possible duplicate of [What are the requirements for C++ template parameters?](http://stackoverflow.com/questions/643763/what-are-the-requirements-for-c-template-parameters) — martin, May 13 '15 at 13:39

score 3 · Accepted Answer · answered May 13 '15 at 13:38

You can not use extern const int as array size because compiler does not know the size of a constant from other compilation unit.

Change your design to use std::vector or some other container to overcome the problem or put the constant in a header and include it before typedefing.

Compilation error when typedef an std::array

1 Answers1