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.

John.

On May 7, 2013, at 3:17 PM, Richard Smith <richard_at_[hidden]> 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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