生成constexpr字符串表，不能生成常量表达式

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];
};

Answer 1

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;
}

Answer 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.