I'm not personally fond of that direction.

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.

Since our names are reserved identifier, an implementation can predefine them and provide a <features> which does nothing.

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 ;-)

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.

John.

On May 7, 2013, at 3:17 PM, Richard Smith <richard@metafoo.co.uk> wrote:

> Hi,
>
> 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:
>
> #ifdef __cpp_lib_header_features
> #include <features>
> #endif
>
> #ifdef __cpp_relaxed_constexpr
> constexpr
> #endif
> size_t strlen(const char *p) { /* ... */ }
>
> ... and so on.

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