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
– helb, Commented May 13, 2015 at 13:39
possible duplicate of What are the requirements for C++ template parameters? — martin
– martin, Commented May 13, 2015 at 13:39

Mohit Jain · Accepted Answer · 2015-05-13 13:38:50Z

3

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.

answered May 13, 2015 at 13:38

Mohit Jain

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