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Subject: Re: [boost] [C++0x] More config macros needed
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-05-13 08:49:53

Beman Dawes wrote:
On Tuesday, May 12, 2009 9:16 PM
> On Tue, May 12, 2009 at 5:11 PM, Stewart, Robert
> <Robert.Stewart_at_[hidden]> wrote:
> > Beman Dawes wrote:
> > On Monday, May 11, 2009 10:15 PM
> >>
> >> Thus we need more config macros, particularly those aimed at the
> >> standard library. A possible approach:
> >>
> >> BOOST_NO_CONCEPTS // needed to tell if headers have been
> >> conceptized
> >>
> >> BOOST_NO_*_HDR // for each new C++0x header; * is
> >> name of header
> >>
> >> For example, BOOST_NO_CHRONO_HDR is defined unless header
> >> <chrono> is present.
> >
> > I noticed the need for such macros, too, at BoostCon. I
> was thinking we need one for each feature.
> We've already got these:
> So we are filling in some of the missing pieces rather than starting
> from scratch.

I never got far enough to look for the config macros you listed; I just noticed that they'd be needed. I'm glad those are available.

> > The only reason I can think of to provide one for each
> header is so that Boost headers can include standard headers
> rather than compatibility code. Is that what you had in mind?
> Yes. Suppliers seem to be adding C++0x library components on a header
> by header basis, rather than waiting until they have a full set and
> then supplying them all at once. Thus the need to have macros for each
> header.
> > If so, then the header macros should only be undefined
> when the corresponding header is complete.
> The problem with waiting for a header to be totally complete means
> waiting a long time. For example, several vendors are already, or will
> soon be, shipping a number of new C++0x headers. But their
> implementations aren't "complete" because they don't have concepts
> yet. The headers are perfectly usable, however, so there is no reason
> not to take advantage of them now. For some of these compilers we many
> have to wait several years before concepts become available, and the
> headers become "complete".

Yes, concepts are unique in coming very late to the party. I agree with your idea, so long as the header macros don't indicate a header is present with any other deficiency or the logic to determine if needed functionality is present will get complicated.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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