comptime/asstring.hpp

The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <string>
#include <format>

// convert an integral value to a std::string
// - can be called at compile time or runtime
constexpr std::string asString(long long value)
{
  if (std::is_constant_evaluated()) {
    // compile-time version:
    if (value == 0) {
      return "0";
    }
    if (value < 0) {
      return "-" + asString(-value);
    }
    std::string s = asString(value / 10) + std::string(1, value % 10 + '0');
    if (s.size() > 1 && s[0] == '0') {  // skip leading 0 if there is one
      s.erase(0, 1);
    }
    return s;
  }
  else {
    // runtime version:
    return std::format("{}", value);
  }
}