better-enums/doc/demo/105-c++17-reflection.md

C++17 reflection proposal

You can try this demo live online.

Better Enums can be used to implement the enums portion of the $cxx17 reflection proposal N4428 in $cxx11. N4428 proposes the following traits interface:

<em>namespace std {

template <typename E>
struct enum_traits {
    struct enumerators {
        constexpr static size_t         size;

        template <size_t I>
        struct get {
            constexpr string_literal    identifier;
            constexpr static E          value;
        };
    };
};

}</em>

So, the basic usage would be:

<em>enum class Foo {A, B, C};

constexpr size_t            size =
    std::enum_traits<Foo>::enumerators::size;

constexpr Foo               value_0 =
    std::enum_traits<Foo>::enumerators::get<0>::value;

constexpr string_literal    name_1 =
    std::enum_traits<Foo>::enumerators::get<1>::identifier;</em>

Resulting in the values 3, Foo::A, and "B", respectively.

---

The interface is implemented in the optional header file extra/better-enums/n4428.h. There are two necessary differences.

1. The interface is only available for enums declared through the BETTER_ENUM macro. This is because the macro is what generates the information necessary for reflection.

Demo

So, with that out of the way, we can do a little test. Let's assume that extra/ has been added as a directory to search for include files.

#ifndef BETTER_ENUMS_CONSTEXPR_TO_STRING
#define BETTER_ENUMS_CONSTEXPR_TO_STRING
#endif

#include <iostream>
<em>#include</em> <<em>enum.h</em>>
<em>#include</em> <<em>better-enums/n4428.h</em>>

---

Let's declare an enum:

<em>BETTER_ENUM</em>(<em>Channel</em>, <em>char</em>, <em>Red</em> = <em>1</em>, <em>Green</em>, <em>Blue</em>)

...and try N4428:

constexpr std::size_t   <em>size</em> =
    <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>size</em>;

constexpr Channel       <em>value_0</em> =
    <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>0</em>>::<em>value</em>;

constexpr Channel       <em>value_1</em> =
    <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>1</em>>::<em>value</em>;

constexpr const char    *<em>identifier_2</em> =
    <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>2</em>>::<em>identifier</em>;

...and check the results:

static_assert(<em>size</em> == <em>3</em>, "");

static_assert(<em>value_0</em> == +<em>Channel::Red</em>, "");
static_assert(<em>value_1</em> == +<em>Channel::Green</em>, "");

int main()
{
    std::cout << <em>identifier_2</em> << std::endl;
    return 0;
}

That prints Blue, as you would expect.

Quirk

The reason for the #define in the code above is that there is one quirk: the interface above is available only for Better Enums for which compile-time name trimming is enabled — those declared when BETTER_ENUMS_CONSTEXPR_TO_STRING was defined, or declared with the SLOW_ENUM variant of BETTER_ENUM. As mentioned on the linked page, the reason compile-time name trimming is not the default is that, while still pretty fast, it is four times slower than program-startup-time name trimming. The latter is the default.

Despite the above, a variation on the interface is available for enums without compile-time name trimming:

<em>namespace std {

template <typename E>
struct enum_traits {
    struct enumerators {
        constexpr static size_t         size;

        template <size_t I>
        struct get {
            constexpr const char        *identifier;
            constexpr static E          value;
        };

        </em>// For enums without compile-time name trimming.<em>
        template <size_t I>
        struct get_alt {
            static const char* identifier();
            constexpr static E          value;
        };
    };
};

}</em>

As you can see, the difference is that identifier is a non-constexpr function, and you have to access it through get_alt<I>.

// Without compile-time name trimming.
<em>BETTER_ENUM(Depth, int, HighColor, TrueColor)

int main()
{
    std::cout
        << std::enum_traits<Depth>::enumerators::get_alt<1>::identifier()
        << std::endl;

    return 0;
}</em>

The future

N4428 is the fourth in a series of revisions: N3815, N4027, N4113, N4428. If there are more revisions that change the proposal for enums, I will try to implement those as well.

%% description = Approximate implementation of N4428 enum reflection based on Better Enums.