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Subject: Re: [SG10] Updates to SD-6
From: Richard Smith (richard_at_[hidden])
Date: 2015-01-09 15:35:01


On Fri, Jan 9, 2015 at 7:19 AM, Ed Smith-Rowland <3dw4rd_at_[hidden]> wrote:

> On 01/08/2015 08:29 PM, Richard Smith wrote:
>
> On Thu, Jan 8, 2015 at 9:25 AM, Nelson, Clark <clark.nelson_at_[hidden]>
> 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.
>

That does not sound entirely sane to me: I think you're suggesting that a
programmer would duplicate some set of numeric literals, just so they could
put digit separators in one copy of them. The risk of the two copies
becoming out-of-sync seems like sufficient justification for any reasonable
programmer to avoid that. Also, consider that #if-guarded code still needs
to successfully tokenize, and literals with odd numbers of digit separators
do not tokenize in C++11 and before.

> > > > 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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