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

On 9/24/2020 4:59 AM, Paul A Bristow via Boost wrote:

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-----Original Message----- From: Boost On Behalf Of Gavin Lambert via Boost Sent: 24 September 2020 03:57 To: boost@lists.boost.org Cc: Gavin Lambert Subject: Re: [boost] Proposal for adding C++ level to the meta/libraries.json

On 23/09/2020 23:15, Rainer Deyke wrote:

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I'm actually very interested in when a library is rendered obsolete by a C++ standard. Lots of Boost libraries have equivalents in the C++ standard library, or in some cases in the language itself. In some cases, the standard library component has completely rendered the Boost version obsolete. In some cases, the Boost version only exists as a backport of the standard library component, and was never intended to be used in C++ versions that include that component. In some cases, the Boost version and the standard library component have developed in different directions, and both are viable. And in some cases, the Boost version exists to correct a perceived flaw in a standard library component, so the Boost version should probably be preferred. It is often not clear which of these applies to which library, even after reading the library documentation (which may predate the standard library component).

In theory, I think that's what the existing "std" field was for: "this library is included in this standard version".

Although that's not the whole story, since e.g. Boost.Assign as you mentioned was not directly included but (AFAIK) is rendered entirely obsolete by initializer lists, and e.g. Boost.SmartPtr and Boost.Thread while *mostly* obsolete do include some extended functionality not in the standard implementation.

And Boost.Variant[2] make different design choices from the standard implementation, so all three are viable alternatives.

I don't think any meta field is really going to capture these sorts of things too well.

But better than nothing?

Provided we don't oversell the information. We need some BIG cautions that this is only a very approximate guide to the full story - which is ultimately "try it and see".

My disgust with the "try it and see" approach is the main reason why I proposed the additions to the meta information, which would, at the very least, tell the end-user whether a Boost library is compilable for the C++ standard level they are using. Is this really too much information to give end-users, and is it really better that an end-user should have to dig through Boost library docs, where the information is often not there at all even if it could be found, just to find out if Boost library X can be compiled by the end-user who is using C++ standard level Y for his project ? I doubt it. Yes, a given library might be more complicated, and provide some functionality over and above the minimal C++ standard level needed to use that library depending on a host of features in the compiler being used, but that is no reason to not at least tell the end-user what the minimum level needed actually is, and then the end-user can read the docs and hopefully find out what other "advanced" library features, needing what other C++ standard level features, are available for that library. But that does not negate the fact that simply using the basic features of a Boost library the end-user would like to know if his C++ standard compilation level is good enough for X library's main functionality.