Re: [boost] [MPL] A Proposal

On 2/29/2016 2:50 PM, Bruno Dutra wrote:

2016-02-29 14:39 GMT-03:00, Edward Diener :

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On 2/29/2016 3:45 AM, Andrey Semashev wrote:

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On 2016-02-29 06:48, Edward Diener wrote:

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On 2/28/2016 7:13 PM, Andrey Semashev wrote:

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On Mon, Feb 29, 2016 at 2:17 AM, Bruno Dutra wrote:

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Dear Community,

[snip]

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My proposal is to make Metal officially into a new revision of Boost.MPL's API, essentially MPL2 as the original proposal by Robert Ramey put it, merging both into one single TMP library. The idea would be to provide a thin proxy for the current Boost.MPL API which would have two backends configurable by preprocessor switches: the original implementation and a another one based on Metal.

I don't think it needs to be controlled with preprocessor switches. In fact, it's best to avoid config macros as much as possible because it complicates the use of the library in other libraries.

I disagree with this. Simply because you are compiling with c++11 on up will not necessarily mean that you want to use the Metal additions to Boost.MPL rather than the current Boost.MPL. Certainly the programmer should have a choice even when C++11 is be used in the compilation.

I don't see a use case where you would want a C++03 implementation on a C++11 capable compiler. On the other hand I can easily see the situation where several libraries using Boost.MPLv2 conflict because they have defined different config macros for it.

You have a good point with your last sentence. The best thing might be to put the MPLv2 header files in its own separate version2 sub-directory structure.

I don’t meant to say you got it wrong, on the contrary I’m certain you got it right, but before we start misunderstanding ourselves, allow me to clarify some terminology so it is clear to everyone.

* Currently Boost.MPL has one API which we all know and love. I call it API v1. * API v1 has currently only one implementation, which is, naturally, that provided by Boost.MPL. * I propose to provide another implementation for API v1 based on Metal, that could be used when the compiler supports C++11. This alternative backend would bring the benefit of faster compilation times and lower memory consumption to older projects.

As long as the Metal API v1 has the exact same syntax, where it exists, as the current MPL, there should be no problem with what you propose. In that case I agree with Andrey that the level of c++, whether c++11 on up or not, can determine whether the current MPL is used or the Metal API v1 is used, for the syntax involved. My concern is simply that current Boost libraries which use MPL, of which there are quite a few as you know, not be forced to change syntactically when Metal is integrated with MPL.

* I also propose to define a new API v2, which is basically that of Metal as it is now, and which would be encouraged for new projects. * Boost.MPL2 in this context would be this new revision of Boost.MPL which has two APIs, v1 and v2, and two implementations for API v1.

Sounds good to me.

I really hope that makes it a bit clearer for everyone.