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

C++17 reflection

Better Enums can be used to approximately implement the enums portion of the $cxx17 reflection proposal N4428 in $cxx11. The implementation is approximate in the following senses:

• It only applies to Better Enums, not built-in enums.
• enum_traits<E>::enumerators::get<I>::identifier is a non-constexpr function rather than a constexpr variable. I could make it a constexpr variable as in the proposal, but that requires compile-time name trimming to be enabled for the Better Enum on which get is used. Since that's an opt-in feature, I can't guarantee it. I preferred not to write feature-detection code, in order to keep the implementation simple.
• The return type of identifier is const char* instead of the proposed std::string_literal, because I don't have an implementation of the latter available. I'm also ignoring the requirements on encoding, and just taking whatever the preprocessor provides.

With that out of the way, we can look at a simple example.

---

The implementation is defined in extra/better-enums/n4428.h. Let's assume that extra/ has been added as a directory to search for include files.

#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>)

N4428 proposes three constexpr traits, of which we have two implemented exactly — that is, as constexpr:

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

<em>constexpr Channel       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>;
<em>constexpr Channel       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>;

Let's 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>, "");

Finally, we can try using identifier:

int main()
{
    std::cout
        << <em>std</em>::<em>enum_traits</em><<em>Channel</em>>::<em>enumerators</em>::<em>get</em><<em>2</em>>::<em>identifier</em>()
        << std::endl;

    return 0;
}

That prints Blue, as you would expect.

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