Feature request: Add helper macros

[hg-zt]

In nlohmann/json we have https://github.com/nlohmann/json#simplify-your-life-with-macros
so that I could simply define both serializer and deserializer in one line:

namespace my_ns {

struct MyType {
  int some_value;
  OtherSerializableType some_obj;
}

NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(MyType, some_value, some_obj);
}

which makes it possible to convert from/to

{
   "some_value": 1,
   "some_obj": { ... }
}

I'd like to have similar macros to generate into_toml and from_toml for simple structs. Something like TOML11_DEFINE_NON_INTRUSIVE.

[ToruNiina]

Sounds nice. I think I can implement it quickly.
But, because of the current implementation, the usage would be slightly different from the code shown.

[ToruNiina]

I'm struggling with the difference in the preprocessor implementation between gcc/clang and MSVC. If you are using gcc/clang, here is the working code. If you are using MSVC, please wait a bit more.

https://github.com/ToruNiina/toml11/blob/auto-conversion-macro/toml/macros.hpp

[glebm]

Looks like Microsoft added a preprocessor flag that enables well-behaved __VA_ARGS__ in the recent versions of their compiler, /Zc:preprocessor:

This option enables a token-based preprocessor that conforms to C99 and C++11 and later standards.

Perhaps toml11 could require the flag to be set?

• Documentation: https://docs.microsoft.com/en-us/cpp/build/reference/zc-preprocessor?view=msvc-160
• VS2019 flag: /Zc:preprocessor
• VS2017 flag: /experimental:preprocessor
• VS2015 and earlier: not available

[ToruNiina]

Thank you very much @glebm ! It works!