Set constexpr variable with non-constexpr function (but is possible to compute in compile time)

Question

header.h

extern constexpr double sqrt_of_2;
extern constexpr double sqrt_of_1_2;
double sqrt(double x);

main.cpp

#include <header.h>

int main() {
  int n;
  scanf("%d", &n);
  printf("%lf %lf\n", sqrt_of_2, sqrt(n));
  return 0;
}

source.cpp

#include <header.h>

double sqrt(double x) {
 // complex bits of math
 // huge function
 // must not be in header for speedy compilation
 // will call other small non-constexpr functions in this file
}

constexpr double sqrt_of_2 = sqrt(2.0);
constexpr double sqrt_of_1_2 = sqrt(0.5)

This obviously does not work.

I can't add constexpr for sqrtin source.cpp because that will not match with declaration in header.h. I also can't add constexpr for sqrt in header.h because constexpr implies inline, I will then need to transfer everything in source.cpp to header.h.

Is this even possible?

CinchBlue · Accepted Answer · 2017-02-26 03:37:52Z

3

No. That's the entire point of why constexpr was created -- to create functions to encapsulate compile-time functions.

It doesn't make sense to compile a compilation unit of code without the compile-time calculations done.

Object files are meant to simply be hooked up to resolve link-time dependencies. Compile-time computations must be defined at compile-time, and, therefore, must have an implementation in the compile-time unit.

answered Feb 26, 2017 at 3:37

CinchBlue

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