C++11: Compile Time Calculation of Array

Question

Suppose I have some constexpr function f:

constexpr int f(int x) { ... }

And I have some const int N known at compile time:

Either<

Answer 1

Boost.Preprocessor can help you. The restriction, however, is that you have to use integral literal such as 10 instead of N (even be it compile-time constant):

#include 

#include 

#define VALUE(z, n, text) f(n)

//ideone doesn't support Boost for C++11, so it is C++03 example, 
//so can't use constexpr in the function below
int f(int x) { return x * 10; }

int main() {
  int const a[] = { BOOST_PP_ENUM(10, VALUE, ~) };  //N = 10
  std::size_t const n = sizeof(a)/sizeof(int);
  std::cout << "count = " << n << "\n";
  for(std::size_t i = 0 ; i != n ; ++i ) 
    std::cout << a[i] << "\n";
  return 0;
}

Output (ideone):

count = 10
0
10
20
30
40
50
60
70
80
90

The macro in the following line:

int const a[] = { BOOST_PP_ENUM(10, VALUE, ~) }; 

expands to this:

int const a[] = {f(0), f(1), ... f(9)}; 

A more detail explanation is here:

• BOOST_PP_ENUM