Peter, your library is great. The structure is extremely useful - being able to generalise operations through absolute and relative types is really nice (e.g. Being able to define a time and a timedelta, such that you can add a timedelta to a time, and subtract two times to get a timedelta, but not being able to add two times).

Matt, if this were to become a language thing, I don't see how the above could be done so easily. I also wonder how it would handle a function declaration that occurs /after/ the "using new" statement. If A and B are both using new X, and a function f(X, X) is declared after the definitions of A and B, is f(A, A) valid? f(A, B)?




On Fri, 21 Feb 2020, 12:16 Matt Hurd via Std-Proposals, <std-proposals@lists.isocpp.org> wrote:
see my "PSSST" (Peter's simple strong typing) framework attempt (not finished) at

https://github.com/PeterSommerlad/Psst/blob/master/cevelop-workspace/PssstTest/src/pssst.h

achievable through a library, so no need to change. In addition it allows you to clearly limit the available operations to a set suitable for your domain specific type. For usage see accompanying test cases.
Thanks Peter. That's a nice approach.

PSSST has similar goals to another couple of libs I've used for safety and strong typing of numerics. However, I'm thinking about a more fundamental thing of cloning a type without it being seen as the same type by the compiler's type system. 

I'm also wondering if such a thing may be useful for also extending things, such as tuples, optional, vector, variant without the potential pitfalls of inheritance. Though different composability customisation points would be useful in such situations (pre, post, invariant, in, out, dot operator, etc). C# has extension methods and we have difficulties without the promising C++3a reflection/rewriting. Unfortunately C++ may fail to optimise properly across abi boundaries for simple composed types which can be frustrating (the infamous unique_ptr example). Performance concerns makes my own simple wrapper types feel a queasy as I'd like to get out of the optimiser's way as much as possible.

Kind regards,

--Matt.
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