strong_type modifier optional

[ksergey]

Firstly thanks for nice library.

Will be good to see optional modifier for a type which will looks like:

using Qty = strong::type<std::size_t, struct Tag_Qty, strong::optional<0>>;

auto qty = Qty(0);
assert(qty.has_value() == false);

qty = Qty(10);
assert(qty.has_value());

in this case 0 mean no value.

Thanks

[rollbear]

I am a little confused here. What is it you would gain over just using std::optional<my_strong_type>?

https://godbolt.org/z/as3jjf1va

[rollbear]

Ah, OK, now I think I see. You want to mark a certain value, 0 in your example, as "not having a value", am I right?

I know this is a frequently used pattern in some domains, but I really don't like it much. It's also more complicated, because in many cases (see many posix functions) all negative values means "no value", so it's tricky to define.

[ksergey]

You want to mark a certain value, 0 in your example, as "not having a value", am I right?

Yes!

I using SBE (Simple Binary Encoding) for messaging between my components. SBE schema could define null value for a type which indicates no value for a field.

[rollbear]

Hmmm. I don't think I will add that to the library. However, I will add a section in the documentation about how to write your own type modifiers, and I will probably use that as an example, because it's a simple one to explain.

[ksergey]

Nice, thank you

[rollbear]

Here's the docs. It uses functionality not yet on main (for the tests, not the implementation).

https://github.com/rollbear/strong_type/tree/v9-dev#writing_modifier

[ksergey]

Super! Thanks!