Meta Enum - Static reflection on enums in C++17

Brief

Single-header facility for compile time reflection of enums in C++17.

Features

Automatic string conversion of enum entries
Tracking of enum size - i.e. member count
Look up enum entries by index
Convenience functions for converting between all of the above
Supports both enum and enum class
Supports enums nested in types/namespaces/functions
Keeps track of the C++ code used to declare the enums
Single header standard C++17 magic.

Compiled Example

See compiled code at: https://godbolt.org/z/TaPqPa

Tested On

gcc 7.3 - works
gcc 8.1 - works
gcc 8.2 - works
clang 6.0 - works - however broken with current stdlib version on godbolt's compiler explorer
MSVC pre 2018 (version on godbolt's compiler explorer) - works for typical case, not for complex enum test case

Installation

Either make sure the meta_enum.hpp file is made available in your include paths or just copy it to your project.

Usage

#include <meta_enum.hpp>

Declaring Enums

Use the macro meta_enum to declare your enums.

meta_enum(Days, int, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday);
//equivalent to:
//enum Days : int { Monday, ...

Or use meta_enum_class for enum class.

The meta_enum macro supports assigning custom values to entries as per usual.

meta_enum(MyEnum, uint8_t, First = 0, Second = 1 << 5, Third = myConstexprFunction<int>());

Meta Enum Objects

By using meta_enum to declare MyEnum, you get the global constexpr object MyEnum_meta which stores a representation of your enum and is of type MetaEnum.

MetaEnum type - contains data for the whole enum declaration and holds all members

Typedefs:

UnderlyingType - contains the selected underlying integer type for the enum

Members:

string - contains the full enum declaration as a string_view
members - contains representations of all enum members represented as an std::array with MetaEnumMember objects

MetaEnumMember type - contains data for one particular enum member

Members:

value - The enum value, for example MyEnum::Second
name - String representation of the member. For example "Second"
string - The whole declaration string. For example " Second = 1 << 5"
index - The numerical index of this member. For example 1

Convenience functions

A few functions are provided per meta_enum to ease usage.

For an enum with the name MyEnum you will get the following:

std::string_view MyEnum_value_to_string(MyEnum) converts an enum value to a textual representation. Will use the .name member of the member, or "__INVALID_ENUM_VAL__" on invalid input.
std::optional<MetaEnumMember> MyEnum_meta_from_name(std::string_view) Accesses the meta object for a member found by name. Returns nullopt on invalid input.
std::optional<MetaEnumMember> MyEnum_meta_from_value(MyEnum) Accesses the meta object for a member found by enum value. Returns nullopt on invalid input.
std::optional<MetaEnumMember> MyEnum_meta_from_index(std::string_view) Accesses the meta object for a member found by enum member index. Returns nullopt on invalid input.

Examples

See the file in the repo examples.cpp

Problems and limitations

Build errors

Some configurations of certain compilers seem to break down when building this. Specifically there is a problem with clang 6.0 on godbolt which fails to build due to what looks like problems with std::string_view not being constexpr even though it should be. See: https://godbolt.org/z/Ob9Cnv There has also been problems when building with the MSVC Pre 2018 hosted @godbolt.org as well when it builds with the complex test case inside of example.cpp. It works however for the typical case. See: https://godbolt.org/z/rIhpfR

Enum parsing problems with nested templates

meta_enum uses string parsing to be able to extract the names of each enum entry. This string parsing takes into account that enum declarations can contain pretty complex meta-programming expressions - it's not as simple as counting commas. The algorithm for doing so is somewhat naive and cannot tell apart >> as the operator and >> as the end of two nested templates (std::vector<std::vector<int>> for example). Due to this, nested template angle brackets have to be separated by whitespace, i.e. >> has to be > >.

Contribution

I'm gladly welcoming suggestions for improvements, pull requests, bug reports, comments or whatever. :)

therocode/meta_enum