Generate constexpr string table, cannot result in a constant expression

Question

Inspired in https://stackoverflow.com/a/37413361/1734357 I wish to make a string color lookup table of a fixed and known size, so I shouldn't need to template it, but string isn't constexpr

How to go about it?

struct Colors
{
    constexpr Colors() : colors()
    {
        for (size_t i = 0; i < 256; i++)
            colors[i] = "0;" + to_string(i) + ";255";
        for (size_t i = 0; i < 256; i++)
            colors[256 + i] = "0;255;" + to_string(255 - i);
        for (size_t i = 0; i < 256; i++)
            colors[2 * 256 + i] = to_string(i) + ";255;0";
        for (size_t i = 0; i < 256; i++)
            colors[3 * 256 + i] = "255;" + to_string(255 - i) + ";0";
    }
    string colors[4*256];
};

You can't use the `std::string` constructor for `constexpr` expressions. — Hatted Rooster, Dec 18 '19 at 13:33
(`std::string_view` is `constexpr`, but not sure how much that helps here.) — Max Langhof, Dec 18 '19 at 13:37
@MaxLanghof, it may help, but can't test it as at present I don't have C++17 — WurmD, Dec 18 '19 at 13:38
@WurmD What C++ version do you have then? 11? 14? 03? Please add the corresponding tag to make sure that any answers actually help you. — Max Langhof, Dec 18 '19 at 13:59
BTW, why only those 1024 and not all the 16M ones? Memory space? — Bob__, Dec 18 '19 at 14:07
Here is a great explanation + library to do this (minus the conversion from int) in C++11: https://akrzemi1.wordpress.com/2017/06/28/compile-time-string-concatenation/ — Max Langhof, Dec 18 '19 at 14:31

score 2 · Answer 1 · edited Dec 19 '19 at 14:10

The problem is that to_string returns std::string, and std::string is not constexpr constructable.

This issue can be solved by using Sprout constexpr library. Although you might be disappointed by the compilation time.

#include <iostream>
#include <sprout/string.hpp>
#include <sprout/array.hpp>
#include <sprout/algorithm/transform.hpp>
#include <sprout/numeric/iota.hpp>

struct colorConv {
  typedef sprout::string<20> result_type;

  constexpr result_type
  operator()(int n) const {
      return n / 256 == 0 ? "0;" + sprout::to_string(255 - n) + ";255"
      : n / 256 == 1 ? "0;255;" + sprout::to_string(2 * 255 + 1 - n)
      : n / 256 == 2 ? sprout::to_string(3 * 255 + 2 - n ) + ";255;0;"
      : "255;" + sprout::to_string(4 * 255 + 3 - n) + ";0";
  }
};

struct Colors {
private:
  typedef colorConv::result_type string;
  constexpr static auto source = sprout::iota<sprout::array<int, 256*4> >(0);

public:
  constexpr static auto colors = sprout::transform<sprout::array<string, 256*4> >(
      sprout::begin(source),
      sprout::end(source),
      colorConv()
  );
};

int main() {
  auto& a = Colors::colors;

  for (auto&& str : Colors::colors) {
    std::cout << str << std::endl;
  }

  return 0;
}

True but he doesn't have to use operator+ because "255;" "NUMBER" ";0" is a valid C++ expression — Schnappi, Dec 18 '19 at 13:48
Literal concatenation and `std::string`? How does this go together? — Scheff's Cat, Dec 18 '19 at 13:49
Yea I meant not using std::string for colors member variable. He can still use the StringBuilder from the link I posted. He will have access to length and easily construct std::string from const char * if he wants to — Schnappi, Dec 18 '19 at 13:52
Though, the template machinery you linked might be sufficient. Scaring... ;-) — Scheff's Cat, Dec 18 '19 at 13:52

score 2 · Answer 2 · answered Dec 18 '19 at 13:56

2

Quite honestly, I'd do this in a Python script, generating a C++ source file (consisting of an array definition) as a pre-build step.

That way, you get the information baked into your application at compile time, and the source for the information is still code so easy to modify.

constexpr is fine but sometimes you just want to auto-generate some code.

answered Dec 18 '19 at 13:56

Lightness Races in Orbit

378,754
76
643
1,055

I once called this generative programming vs. generic programming. ;-) AWK is yet another tool which is useful for preprocessing allowing source code which consists of tab-separated text files (prepared in Excel). – Scheff's Cat Dec 18 '19 at 15:45

Generate constexpr string table, cannot result in a constant expression

2 Answers2