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 optional Better Enums header file extra/better-enums/n4428.h implements this interface, though with two necessary differences.

1. The interface is only available for Better Enums, i.e. enums declared through the ENUM macro. This is because the macro is what generates the information necessary for reflection.
2. The type of identifier is const char*, not the ${cxx17} proposed string_literal.

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>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.

---

So, Better Enums can be used in $cxx11 to test N4428, and maybe start developing libraries on top of it. N4428 is very low-level, so it needs additional code over it to be truly useful. Whether developing now is a good idea is debatable, since N4428 is still only a proposal. But, it's an interesting thing to consider.

Also, Better Enums can be implemented almost completely in terms of N4428, so the two interfaces are in some vaguely mathematical sense "equivalent." If N4428 is accepted, I may implement a variant of Better Enums on top of it, since it will remove many limitations.

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, i.e. those declared when BETTER_ENUMS_CONSTEXPR_TO_STRING was defined, or declared with the SLOW_ENUM variant of 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>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.