Re: Proposal for adding C++ level to the meta/libraries.json

From: pbristow_at_[hidden]
Date: 2020-09-22 08:42:06

> -----Original Message-----
> From: Boost <boost-bounces_at_[hidden]> On Behalf Of Edward Diener via Boost
> Sent: 21 September 2020 18:37
> To: boost_at_[hidden]
> Cc: Edward Diener <eldiener_at_[hidden]>
> Subject: [boost] Proposal for adding C++ level to the meta/libraries.json
> 
> Since we are reviewing Vinnie Falco's JSON library I thought that this would be a good time to present a
> proposal to add information to each Boost library's meta/libraries.json file regarding the level of C++
> standard compliance for that library. This could then be added to each library's visual documentation so
> that end-users would instantly know the C++ standard level they would need to use in order to use a
> Boost library.
> 
> My proposal is to add 3 fields whose data would be the same as the cxxstd allowed values from Boost
> Build, using the first value of each choice ( currently 03, 11, 14, 17, 20 ).
> 
> standard = minimum C++ level for the library extended = same functionality in the library as the
> minimum C++ level but with extended use given higher C++ levels required = new functionality in the
> library above the minimum C++ level which requires given higher C++ levels
> 
> The 'standard' field would be a single value, while the 'extended' or 'required' fields could be more than
> one comma separated value. If you don't like the names for 'standard', 'extended', or 'required' you can
> bikeshed the name, although I think the 'standard' name is pretty well apparent.
> 
> The idea is to provide this information in the meta/libraries.json field for each library, so that the end-
> user of the library can immediately know the usability of the library with appropriate C++ standard
> levels of compilation.
> 
> I realize that the line between the 'extended' and 'required' entries, as I have described them, could be
> blurred. The 'standard' field remains the most important. I have long felt that the end-user should not
> have to do any investigation, in code or in documentation, just to determine if a library is usable for his
> C++ standard level of compilation.

I've been muttering about this tricky issue for some time.

For a big and old and still growing library like Boost.Math, the problem is even more difficult.

For many math functions and distributions, C++03 (with C++11 numeric_limits) is fine, but as one starts to use newer items, then requirement shift up to needing C++20.

We have recently decided to require a minimum C++11, but that still means that users may find that some components need more.

Predicting the requirements for certain is difficult -  I fear in practice, it means 'sucking and seeing' ☹

I have suggested marking each header file with its minimum requirements, but that is messy and doesn't really help the user much.

My ideal is to say 'always use the latest compiler version and highest standard level', but that isn't popular 😉

But it is a reason to encourage users to plan to keep updating all their tools - and keep their management on-board with that concept.

So I support Edward's proposal, but caution that it isn't as helpful for some libraries as others.

The bottom line is always going to be: "If it works for your configuration, fine - otherwise bad luck."

Paul