Maps

Welcome to the Better Enums tutorials! The code in this tutorial forms a valid program, which you can download and play with. The program runs as part of the automated test suite.

It is possible to create constexpr bidirectional maps between Better Enums and any type. This is currently an experimental feature. Feedback is very much wanted, but please don't build any mission-critical code on top of this :)

The way it works is you give Better Enums a function — say, const char* describe(Channel). The library enumerates it to make a map.

The reason for using a function is that a switch statement is, I believe, the only place where a compiler will check for exhaustiveness. If you forget to create a case for one of the enum's constants, the compiler can let you know. Obviously, a switch statement is not data, and needs to be inside a function. It can only be inside a constexpr function in C++14, so this feature is most natural in C++14. When you pass the function to Better Enums, the library can build up a lookup data structure at compile time.

Actually, right now, Better Enums doesn't quite do that — it enumerates the function every time you want to convert to an enum (but not from an enum). It simply does a linear scan every time. This is because I haven't yet found a data structure whose compile-time generation is fast enough for practical use.

#include <iostream>
#include <enum.h>

BETTER_ENUM(Channel, int, Red, Green, Blue)

We will create a map from this function:

constexpr const char* describe(Channel channel)
{
    switch(channel) {
        case Channel::Red:   return "the red channel";
        case Channel::Green: return "the green channel";
        case Channel::Blue:  return "the blue channel";
    }

    return "needed for gcc 5";
}

Here is the map. The actual type is better_enums::map<Channel, const char*>.

constexpr auto descriptions = better_enums::make_map(describe);

And the usage:

int main()
{
    std::cout << descriptions[Channel::Red] << std::endl;

    std::cout << descriptions.from_enum(Channel::Red) << std::endl;
    std::cout << descriptions.to_enum("the green channel") << std::endl;

    auto not_a_literal = std::string("the blue channel");
    std::cout << descriptions.to_enum(not_a_literal.c_str()) << std::endl;

    return 0;
}

make_map above produces a value of type better_enums::map<E, T>. The full signature of the template better_enums::map is

template <typename Enum, typename T, typename Compare = map_compare<T>>

Compare has to be a class with a static member function bool less(const T&, const T&). The default implementation better_enums::map_compare simply applies operator <, except when T is const char* or const wchar_t*. In that case, it does lexicographic comparison.

Better Enums

Reflective compile-time enums for C++

Open-source under the BSD license

Maps