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From: Edward Diener (eldiener_at_[hidden])
Date: 2020-09-22 16:02:39

On 9/22/2020 4:42 AM, Paul A Bristow via Boost wrote:
>> -----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.

You could tag Math as:

standard : 11
required : 14, 17, 20

It least that tells end-users that C++11 is the minimum level but that
some functionality will need C++14, C++17, or C++20. I think that
information is very helpful to end-users. Nothing is perfect but let's
settle for what is at least good and helpful.

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

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