constexpr enum map

Associate arbitrary constexpr values to enum values.

#include <constexpr_enum_map.hpp>
#include <string_view.hpp>

enum Fruit : uint32_t {
  Apple,
  Cherry,
  Lychee,
  Mango,
  Prune,

  DoNotUseMeMustBeLastEnumVariant
};

struct FruitProperty {
  Fruit _enum;
  bool soft;
  int32_t average_weight;
  std::string_view name;
};

DEFINE_CONSTEXPR_ENUM_MAP_TYPE(
  FruitProperty,
  FruitProperty{Fruit::Apple, false, 120, "apple"},
  FruitProperty{Fruit::Cherry, true, 4, "cherry"},
  FruitProperty{Fruit::Lychee, true, 20, "lychee"},
  FruitProperty{Fruit::Mango, true, 200, "mango"},
  FruitProperty{Fruit::Prune, true, 55, "prune"}
);

static constexpr auto fruit_properties = FruitProperty_enum_map_t::value();

auto is_heavy_soft_fruit(const Fruit fruit) -> bool {
  const auto fruit_property = fruit_properties[fruit];
  return fruit_property.soft && fruit_property.average_weight > 50;
}

int main(int argc, char* argv[])
{
  if (argc != 2) {
    return 1;
  }

  // Bounds check becuase of user provided index.
  std::cout << fruit_properties.at(std::stoi(argv[1])).name << '\n';
}

Design Goals

• Easy to use correctly -> produces std::array, intuitive semantics
• Hard to use incorrectly -> compile time property checking
• Excellent code gen -> https://godbolt.org/z/WtV2aw
• Little compile time impact -> no TMP recursion
• Minimal static init work -> constexpr value in static section

• ~10x more binary bloat
• Much worse code gen
• No guarantee that all enum variants have a mapping
• Much more static init work, including heap allocations

When does it not make sense to use this

• If the associated value is only a simple integral -> Use enum values
• If the mapping needs to be modified at runtime -> Use an unordered_map

Compile time impact

TODO

Options

• Some C++14 std library functionality is backported, if you want to use the std versions of them configure your compiler to declare itself at least C++14 compatible and define CEMNOSTD_SHIM, clang example clang -std=c++14 -DCEM_NO_STD_SHIM.

Getting Started

• Include the constexpr_enum_map.hpp header

Prerequisites

• C++11 compiler
• constexpr_enum_map.hpp found in include paths

Installing

• Download the file eg. wget https://raw.githubusercontent.com/Voultapher/constexpr-enum-map/master/constexpr_enum_map.hpp

Running the tests

TODO

Contributing

Please read CONTRIBUTING.md for details on our code of conduct, and the process for submitting pull requests to us.

Versioning

We use SemVer for versioning. For the versions available, see the tags on this repository.

Authors

Lukas Bergdoll - Initial work* - Voultapher

See also the list of contributors who participated in this project.

License

This project is licensed under the Apache License, Version 2.0 - see the LICENSE.md file for details.