The number of elements in an array is part of the array’s type. As a result, the dimension must be known at compile time, which means that the dimension must be a constant expression (§ 2.4.4, p. 65)

unsigned cnt = 42; // not a constant expression
constexpr unsigned sz = 42; // constant expression
// constexpr see § 2.4.4 (p. 66)
int arr[10]; // array of ten ints
int *parr[sz]; // array of 42 pointers to int
string bad[cnt]; // error: cnt is not a constant expression
string strs[get_size()]; // ok if get_size is constexpr, error otherwise