On Tue, May 7, 2013 at 1:00 PM, John Spicer <span dir="ltr"><<a href="mailto:jhs@edg.com" target="_blank">jhs@edg.com</a>></span> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
I'm not personally fond of that direction.<br>
<br>
At least for EDG, and I suspect others, the set of feature supported is going to depend on command-line options, which would mean that we'd have to define one set of macros (or something) to allow the <features> header to define another set. It also makes it more likely that you could have a <features> file that doesn't actually match what the compiler does.<br>
</blockquote><div><br></div><div>Since our names are reserved identifier, an implementation can predefine them and provide a <features> which does nothing.</div><div><br></div><div>This allows feature test macros to be provided (a) with no compiler changes, (b) retroactively for an implementation which was released prior to our recommendations (provide a <features> which checks the compiler and version, and use -D__cpp_lib_header_features), and (c) with (essentially) zero cost for programs which do not use the macros. This also happens to be standardizing existing practice ;-)</div>
<div><br></div><div>That said, I agree with Clark that quantifying the cost in (c) would be useful. I have informally heard that this cost is measurable (especially for things like configure scripts which build a large number of tiny programs), but I've never seen numbers.</div>
<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
John.<br>
<div><div class="h5"><br>
On May 7, 2013, at 3:17 PM, Richard Smith <<a href="mailto:richard@metafoo.co.uk">richard@metafoo.co.uk</a>> wrote:<br>
<br>
> Hi,<br>
><br>
> Has any thought been given to putting the feature-test macros into an implementation-supplied header, instead of predefining them? This would allow us to remove the cost associated with predefining these macros, for translation units which don't need them. Instead, we could supply a single predefined macro indicating whether the header is available, and user code would write something like:<br>
><br>
> #ifdef __cpp_lib_header_features<br>
> #include <features><br>
> #endif<br>
><br>
> #ifdef __cpp_relaxed_constexpr<br>
> constexpr<br>
> #endif<br>
> size_t strlen(const char *p) { /* ... */ }<br>
><br>
> ... and so on.<br>
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<br>
</blockquote></div><br>