Enum-Flag Default Values

[PapeCoding]

Currently I try to use reflect-cpp to serialize a struct to json and back.

enum class PlatformFlag
{
	NONE = 0,
	LINUX = 1 << 0,
	WINDOWS = 1 << 1,
	MAC = 1 << 2,
	ALL = LINUX | WINDOWS | MAC
};
constexpr PlatformFlag operator&(PlatformFlag const a, PlatformFlag const b);
constexpr PlatformFlag operator|(PlatformFlag const a, PlatformFlag const b);
inline PlatformFlag& operator|=(PlatformFlag& a, PlatformFlag const b);

[...]

struct myStruct
{
	PlatformFlag supportedPlatforms = PlatformFlag::NONE;
};

Serializing to json without setting a value yields something like:

{
    "supportedPlatforms": ""
}

and reading it back then errors out with: Invalid enum value: ''. Must be one of [NONE, LINUX, WINDOWS, MAC]..

Am I missing something? Ideally it would either serialize to "NONE" or allow the empty string here.

[liuzicheng1987]

@PapeCoding what version are you using? I am pretty sure we have seen this before and fixed it already.

[liuzicheng1987]

@PapeCoding I cannot reproduce the issue on the newest version of reflect-cpp. Here is the test I wrote:

enum class PlatformFlag {
  NONE = 0,
  LINUX = 1 << 0,
  WINDOWS = 1 << 1,
  MAC = 1 << 2,
  ALL = LINUX | WINDOWS | MAC
};

constexpr PlatformFlag operator&(PlatformFlag const a, PlatformFlag const b) {
  return static_cast<PlatformFlag>(static_cast<uint64_t>(a) &
                                   static_cast<uint64_t>(b));
}

constexpr PlatformFlag operator|(PlatformFlag const a, PlatformFlag const b) {
  return static_cast<PlatformFlag>(static_cast<uint64_t>(a) |
                                   static_cast<uint64_t>(b));
}

inline PlatformFlag& operator|=(PlatformFlag& a, PlatformFlag const b) {
  a = a | b;
  return a;
}

struct myStruct {
  PlatformFlag supportedPlatforms = PlatformFlag::NONE;
};

TEST(json, test_enum9) {
  myStruct t{.supportedPlatforms = PlatformFlag::NONE};

  write_and_read(t, R"({"supportedPlatforms":"0"})");
}

The test passes just fine. The same issue was already addressed with this commit: 498d9d0

So all you need to do is to update to reflect-cpp v0.25.0 and the problem is fixed.

[liuzicheng1987]

@PapeCoding if you have upgraded to the newest version of reflect-cpp and it still doesn't work, feel free to reopen the issue and then please tell me exactly what compiler and OS you are using, so I can investigate further.

[PapeCoding]

@liuzicheng1987 Thanks for your fast help! I did not notice that I was still using v0.24.0 and with the new one the problem is gone. I still think it is weird, that the "NONE" value gets serialized as "0" is this case, but I am happy that now my application can handle this case more gracefully.

[liuzicheng1987]

@PapeCoding this is due to the current limitations that C++ imposes on us, and it is documented:

https://rfl.getml.com/enums/#flag-enums

The problem is that 0 is neither 2^N nor can it be expressed as bitwise ORs of numbers that are.

However, once C++ 26 reflection is fully supported by the compilers, we will have no such limitatation.

[ZXShady]

@PapeCoding this is due to the current limitations that C++ imposes on us, and it is documented:

https://rfl.getml.com/enums/#flag-enums

The problem is that 0 is neither 2^N nor can it be expressed as bitwise ORs of numbers that are.

However, once C++ 26 reflection is fully supported by the compilers, we will have no such limitatation.

@liuzicheng1987

This is not a limitation- enchantum does reflect the zero value by default all you need to do is replace this line

https://github.com/getml/reflect-cpp/blob/main/include%2Frfl%2Fenums.hpp#L74-L76

with

if constexpr(enchantum::has_zero_flag<E>
      return enchantum::names<E>[0];
else
      return "0";

and the issue will be fixed

[liuzicheng1987]

@ZXShady the issue is already fixed using pretty much exactly the solution you proposed.

By the way, I have recently added support for C++-26.

[ZXShady]

@ZXShady the issue is already fixed using pretty much exactly the solution you proposed.

I don't see the solution as what I suggested, it seems the code right now just outputs the literal 0 always whether it has a string identifier or not, I don't think the user above wanted that. From my understanding they wanted it to output "NONE" not "0" if the flag is "0"

By the way, I have recently added support for C++-26.

That's awesome - I added one as well in an enchantum branch, I didn't merge to main as I am still thinking about the api but when that gets merged I will let you know along with the conan package (still pending approval)