Defining constexpr static data members

In

int main() { const int &cs = test::stc; } 

test::stc is odr-used while in

int main () {int array[test::stc];}  

it is not.

The following example from the C++11 Standard supports the above idea.

struct S { static const int x = 0; };
const int &f(const int &r);  
int n = b ? (1, S::x)    // S​::​x is not odr-used here
          : f(S::x);     // S​::​x is odr-used here, so a definition is required

Looking at it from practical point of view, cs will be an invalid reference unless test::stc has an address. array, on the other hand, needs just the value of test::stc, which can be evaluated at compile time. array does not need the address of test::stc to be a valid object.

An object that is odr-used must be defined exactly once in a program.

static constexpr int stc = 1; // declares the static var

constexpr int test::stc; // defines the static var

for more detailed explanation check link below

http://www.learncpp.com/cpp-tutorial/811-static-member-variables/