On 01/08/2015 08:29 PM, Richard Smith wrote:
On Thu, Jan 8, 2015 at 9:25 AM, Nelson, Clark <[email protected]
<mailto:[email protected]>> wrote:
> > > N4051 Allow typename in a template template parameter
> > > __cpp_typename_in_template_template_parm 201411
> > > This may be too little to mess with.
> > This is a really good question. The only way I could see this
being used
> > would be like so:
> > #if __cpp_typename_in_template_template_parm
> > #define TTP typename
> > #else
> > #define TTP class
> > #endif
> > with template template parameters declared like so:
> > template<template<...> TTP X> ...
> > Obviously, the interesting questions are, what sort of
spelling might
> > actually be used for the name of what I have called "TTP", and
would anyone
> > actually bother to write code like this?
> It really depends what we think feature test macros are for. Another
> possible use is this:
> #if !__cpp_typename_in_template_template_parm
> #error You need a compiler supporting C++17 to build this code.
> #endif
> So, do we only care about feature-test macros that allow a
program to use
> the feature if present and reasonably fall back if not, or do we
also care
> about cases where the only reasonable response to a missing
feature is to
> cause an error (or fail a configure check or similar)? The
latter case
> covers this feature, trigraph removal, digit separators, and
probably some
> others, which should presumably be treated uniformly.
For my money, if the only plausible use of a feature-test macro
would be to
control a #error directive, that's not enough justification to
create the
macro. Here's how SD-6 already says it:
(The absence of a tested feature may result in a program with
decreased
functionality, or the relevant functionality may be provided in a
different
way. A program that strictly depends on support for a feature can
just try
to use the feature unconditionally; presumably, on an
implementation lacking
necessary support, translation will fail.)
It's possible that we have already invented some macros that I
wouldn't
really consider to be adequately justified. Nobody's perfect. :-(
That's part of the reason we don't try to put our recommendations
in the
standard itself.
OK, I think this is an entirely reasonable position. On that basis, I
think we do not want a macro for N4051. (I think we also don't want a
__cpp_digit_separators macro; how do we feel about removing it from
our recommendations?)
I think we all agree that there's no sane use for a macro for template
template typename.
On the other hand I can maybe see surrounding blocks of constants with
the digit separator macro.
Then dropping the unseparated block as the last platform implements
digit seps.
> > > N4268 - Allow constant evaluation for all non-type template
arguments
> > > __cpp_const_eval_of_non_type_template_args
>
> > I have tweaked the spelling of this slightly:
> > __cpp_const_eval_all_nontype_template_args
> That seems quite verbose. How about:
> __cpp_nontype_template_arg_eval
> Even then, I worry that the "eval" is missing the point. The
change removes
> a syntactic restriction as much as it introduces different
semantics. I
> wonder if simply
> __cpp_nontype_template_args == 201411
> would be acceptable; I think that's my preferred spelling for this.
OK, thanks.
Clark
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