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

From: Tom Honermann <tom_at_[hidden]>
Date: Wed, 10 Jul 2019 15:05:59 -0400
On 7/10/19 2:02 PM, Gabriel Dos Reis wrote:
> On Jul 10, 2019, at 10:49 AM, Tom Honermann <tom_at_[hidden]
> <mailto:tom_at_[hidden]>> wrote:
>> 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
>> <https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Feel.is%2Fc%2B%2Bdraft%2Fmodule.import%235&data=02%7C01%7Cgdr%40microsoft.com%7Ce1d9e7eedab84332db4308d7055eec54%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636983777563207334&sdata=WwjHx%2FXWe7kf6M5xVLmysV0qIgvxUCR3Ck2A9zKrBd8%3D&reserved=0>,
>> it "is a member of an implementation-defined set of headers". If
>> <foo> is an importable header, then, per [cpp.include]p7
>> <https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Feel.is%2Fc%2B%2Bdraft%2Fcpp.include%237&data=02%7C01%7Cgdr%40microsoft.com%7Ce1d9e7eedab84332db4308d7055eec54%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636983777563217045&sdata=Q4jzXEipiT8GniJM3z8fb6DT5k9vWvHvodq%2F9h5Y4mg%3D&reserved=0>,
>> "the preprocessing directive is instead replaced by the
>> preprocessing-tokens 'import header-name ;'" Headers that are
>> importable headers are not textually included.
> You are implying that the include translation is mandatory. Is that
> your argument?

Yes. The wording in [cpp.include]p7
seems fairly clear regarding that.


Received on 2019-07-10 14:07:54