On 8/28/24 12:32 AM, Jeremy Rifkin via Std-Proposals wrote:
>> Another question is whether the comparison should be post translation
>> phase 1.
> I gave this some thought while drafting the proposal. I think it comes
> down to whether the intent is single inclusion of files or single
> inclusion of contents.

Indeed. The proposal currently favors the "same contents" approach and
offers the following wording.

    A preprocessing directive of the form
    # pragma once new-line
    shall cause no subsequent #include directives to perform replacement
    for a file with *text contents identical to this file*.

The wording will have to define what it means for contents to be
identical. Options include:

  * The files must be byte-for-byte identical. This makes source file
    encoding observable (which I would be strongly against).
  * The files must encode the same character sequence post translation
    phase 1. This makes comparisons potentially expensive.

Note that the "same contents" approach obligates an implementation to
consider every previously encountered file for every #include directive.
An inode based optimization can help to determine if a file was
previously encountered based on identity, but it doesn't help to reduce
the costs when a file that was not previously seen is encountered.

Tom.

>
> Jeremy
>
> On Tue, Aug 27, 2024 at 3:39 PM Tom Honermann via Std-Proposals
> <std-proposals_at_[hidden]> wrote:
>> On 8/27/24 4:10 PM, Thiago Macieira via Std-Proposals wrote:
>>> On Tuesday 27 August 2024 12:35:17 GMT-7 Andrey Semashev via Std-Proposals
>>> wrote:
>>>> The fact that gcc took the approach to compare file contents I consider
>>>> a poor choice, and not an argument to standardize this implementation.
>>> Another question is whether a byte comparison of two files of the same size is
>>> expensive for compilers.
>>>
>>> #once ID doesn't need to compare the entire file.
>> Another question is whether the comparison should be post translation
>> phase 1. In other words, whether differently encoded source files that
>> decode to the same sequence of code points are considered the same file
>> (e.g., a Windows-1252 version and a UTF-8 version). Standard C++ does
>> not currently allow source file encoding to be observable but a #pragma
>> once implementation that only compares bytes would make such differences
>> observable.
>>
>> Tom.
>>
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