Provide a unified API for reading and writing values of variants / nodes

[chandryan]

Hi @lukasberbuer !

I would like to contribute to open62541pp with a simplified API proposal for reading and writing node values 😄 .

I set up some demo code, please take a look at https://github.com/chandryan/open62541pp/pull/1 and tell me if you're interested in it. If yes, I will change the the PR to point to your repo and continue my work.

Kind regards

[lukasberbuer]

Great idea @chandryan! This could also result in a universal Variant constructor instead of Variant::fromValue. As I noticed in your code, the main problem is to differentiate between a container and a string-like type. I can't think of a generic trait, that could separate for example a Span<int8_t> from a std::string_view. BUT we might use the TypeRegistry/TypeConverter with their traits detail::IsRegisteredType/detail::IsConvertibleType: If the provided template type is a registered or convertible type, is has to be a scalar type. There are already template specializations for string-like types:

• opcua::String
• std::string_view
• std::string
• const char*
• char[N]
• opcua::ByteString

I'm looking forward to see the results 😊

[chandryan]

Great! I will try it out if I can make it work :) Thanks for the hint with detail::IsConvertibleType, I will see if I can use that to get rid of the explicit check for STL containers 😄