Preprocessor enumeration macros in C and C++

enumeration-macros.md

Here's a trick to avoid repeating a list of things in multiple places:

#define ENUMERATE_THINGS \
    ENUMERATE_THING(Foo) \
    ENUMERATE_THING(Bar) \
    ENUMERATE_THING(Baz)

#define ENUMERATE_THING(thing) thing
ENUMERATE_THINGS
#undef ENUMERATE_THING

The code above will be expanded by preprocessor into this:

Foo
Bar
Baz

I often use this to declare large enums and making a stringification function for them like this:

enum class Thing {
#define ENUMERATE_THING(thing) thing,
ENUMERATE_THINGS
#undef ENUMERATE_THING
};

const char* to_string(Thing thing)
{
    switch (thing) {
    #define ENUMERATE_THING(thing) case Thing::thing: return #thing;
    ENUMERATE_THINGS
    #undef ENUMERATE_THING
    }
}

The code above will be expanded by the preprocessor to:

enum class Thing {
    Foo,
    Bar,
    Baz,
};

const char* to_string(Thing thing)
{
    switch (thing) {
    case Thing::Foo: return "Foo";
    case Thing::Bar: return "Bar";
    case Thing::Baz: return "Baz";
    }
}

This allows you to keep the canonical list of things in a single place, so you don't have to update multiple functions when adding a value.

(Note: The expanded code will actually look a bit different, whitespace-wise. The C/C++ parser doesn't care though, so I've made it look nice here for demonstration purposes.)