ARROW-13296: [C++] Provide a reflection compatible enum replacement

[bkietz]

Provides an enum replacement with minimal reflection capabilities. These can be declared by inheriting from a helper with CRTP, including a static string literal data member containing the enum's values:

struct Color : EnumType<Color> {
  using EnumType::EnumType;
  static constexpr char* kValues = "red green blue";
};

Values of enumerations declared in this way can be constructed from their string representations at compile time, and can be converted to their string representation for easier debugging/logging/and less repetitive boilerplate in the bindings and elsewhere mapping to/from user provided string values.

For example:

int get_hex_value() {
    std::string input;
    std::cin >> input;
    switch (*Color(repr)) {
      case *Color("red"):
        return 0xff0000;
      case *Color("green"):
        return 0x00ff00;
      case *Color("blue"):
        return 0x0000ff;
      default:
        std::cout << "Don't know that one; input hex value\n";
        std::cin >> std::hex >> input;
        return std::stoi(input);
    }
}

[github-actions]

https://issues.apache.org/jira/browse/ARROW-13296

[lidavidm]

I left some comments, but functionally, this looks good, and we can file a followup to simplify FunctionOptions with this.

[bkietz]

I recommend avoiding macros whenever possible, but for completeness I'll note that we could replace

// color.h
struct Color : EnumType<Color> {
  using EnumType::EnumType;
  static constexpr const char* kValues = R"(red green blue)";
};
// color.cc
constexpr const char* Color::kValues;

with

ARROW_ENUM(Color, red green blue);

[lidavidm]

Quick question, why did 'try replacing an enum' ultimately stay reverted? Did you just mean to leave it as a followup?

[bkietz]

I can certainly replace one in this PR but I wasn't sure that'd be preferred. I was mostly using it as a test to ensure that the C++ builds could all pass with the replacement- if we want to include a replacement here then I'll need to repair the bindings as well

[lidavidm]

Ah nah just wanted to make sure of the intent. Let's merge this? I'll file a JIRA to adjust the bindings and definitions.

[lidavidm]

ARROW-13307

[bkietz]

I want to provide one more tweak to make it easier to enable_if and provide a doccomment

[bkietz]

but I can just append that to the follow up

[lidavidm]

If you wanna do that here that's no problem

[lidavidm]

Thanks for working this out.