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Re: [SG15] [isocpp-modules] Determining identity like #pragma once

From: Tom Honermann <tom_at_[hidden]>
Date: Wed, 10 Jul 2019 13:49:13 -0400
On 7/10/19 1:23 PM, Gabriel Dos Reis wrote:
>> On Jul 10, 2019, at 10:18 AM, Tom Honermann <tom_at_[hidden]> wrote:
>>> On 7/10/19 1:00 PM, Gabriel Dos Reis via Modules wrote:
>>>> On Jul 10, 2019, at 6:27 AM, Bryce Adelstein Lelbach aka wash <brycelelbach_at_[hidden]> wrote:
>>>> On Wed, Jul 10, 2019 at 2:48 AM Gabriel Dos Reis via SG15
>>>> <sg15_at_[hidden]> wrote:
>>>>> Also, we should note that import and #include of the same file can lead to unexpected results, so we need both clarifications and recommendations there for users.
>>>> Why should this lead to unexpected results?
>>>> Suppose I have:
>>>> #include <foo>
>>>> import <foo>;
>>> To illustrate the concern, let me further add that the header <foo> has include guard and all that.
>>> The #include copy-n-pastes the content of foo in the current translation - including of course any definitions. The import makes available any declaration/definition from the translation unit of foo. The program ends up with multiple definitions (at link time at least).
>> This doesn't sound correct to me.
>> For <foo> to be importable at all implies, to me, that the implementation must be aware (somehow) that <foo> designates an importable header. In other words, in this scenario, no copy-n-paste inclusion occurs.
>> [module.import]p5 (https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Feel.is%2Fc%2B%2Bdraft%2Fmodule.import%235&amp;data=02%7C01%7Cgdr%40microsoft.com%7C627c2e697d3246f69cd008d7055aaa49%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C636983759279843021&amp;sdata=KNmreH81AE1GNzLjcAEgzCneqRsmQy%2BhWsuAdwN64qU%3D&amp;reserved=0):
>> ... An importable header is a member of an implementation-defined set of headers ...
> Please expand on how it doesn’t sound correct. All I said follows exactly the spec.

For <foo> to be an importable header, per [module.import]p5
<http://eel.is/c++draft/module.import#5>, it "is a member of an
implementation-defined set of headers". If <foo> is an importable
header, then, per [cpp.include]p7
<http://eel.is/c++draft/cpp.include#7>, "the preprocessing directive is
instead replaced by the preprocessing-tokens 'import header-name ;'"
Headers that are importable headers are not textually included.


Received on 2019-07-10 12:51:08