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`enum_traits` and `enumerator_traits`

`enum_traits` for properties of C / C++ enum types

C++17. Targets GCC & Clang (MSVC is WIP). Namespace ltl.

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  ltl::is_scoped_enum<T>;    // Test if type T is a scoped enum (lazy).
  ltl::is_scoped_enum_v<T>;  // Test if type T is a scoped enum (eager).

  ltl::is_fixed_enum<T>;     // Test if type T is a 'fixed' enum,
  ltl::is_fixed_enum_v<T>;   // i.e. an enum  with fixed underlying type.

  ltl::underlying_type<T>;   // c++17 port of c++20's improved UB-free
  ltl::underlying_type_t<T>; // 'SFINAE-friendly' std::underlying_type.

  ltl::to_underlying(e);     // Convenience cast to underlying type P1682.

`enumerator_traits` for reflection of enumerated values

C++17. Targets GCC>=9 and Clang (recent MSVC possible). Namespace ltl.

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  ltl::is_enumerated_v<e>; // Test if enum value e corresponds to an
                           // enumerator of its enum type decltype(e).

  ltl::enumerators_v<E>;   // Array of enumerated values of enum type E
                           // (see note on duplicate-valued enumerators).
  ltl::enumerators_t<E>;   // integer_sequence<underlying_type_t<E>,u...>
                           // of E's enumerators' underlying values u...
  ltl::enumerators<E>;     // Trait class for enumerated values of E with
                           // array 'value' and typedef 'type' members.

The array returned by enumerators_v is of an internal type, sufficient to index,
range-for iterate or copy into another container.

Description

Two header-only libraries of 'type traits'; template-based utilities to query:

enum_traits Properties of enum types
enumerator_traits Enumerated values of an enum type

The traits interface is constexpr; runtime facilities can be built on top.
These libraries deal only with enumerated values, not with names of ids.
Reflection of names can be dealt with at a higher level.

See the example repo for full reflection of enum types, including names.

Reflection of enumerated values

A 'brute force' method is used to check possible values of the underlying type.
It takes around a second to exhaustively check all 2^16 values of a 16-bit enum
(on a decent recent dev box - GCC faster than Clang, MSVC out of the game).

Support for `underlying_type` bit-widths

Enum types with <= 16-bit underlying type are fully supported - exhaustive check
Enum types with 32-bit underlying type are partially supported - far from exhaustive.

32-bit underlying type partial support

2^16 values of the same-signed 16-bit integer are checked, plus:

for uint32, check 392 high values (UINT16_MAX, UINT32_MAX].
for int32, check 32 negative values [INT32_MIN, INT16_MIN)
- and 360 high positive values (INT16_MAX, INT32_MAX].

The 32-bit Int_MAX high ranges comprise all values with a contiguous run of set bits.
This means that 'flag enums' with contiguous bitfield / bitmask values are supported.

Enum types with 64-bit underlying type are unsupported, giving a compile error.

Platform support

GCC>=9 is required for a bugfix (earlier versions can be patched).
MSVC is work in progress. 8-bit underlying value works.
C++17 for auto template parameters, constexpr if, inline variables...

Disclaimers

Enumerator extraction uses non-standard 'pretty function' preprocessor
extensions whose output differs between compilers & compiler versions.
This is not a future-proof solution. Use with caution.
Test for your use-case and target platforms.

Build

Meson build script provided, e.g. use with ninja backend

meson build
ninja -C build

Linux Travis	Windows Appveyor
gcc-9, clang-7<br>-std=c++17	MSVC 19.21.27702.2<br>/std:c++latest