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Subject: Re: Undefining a compiler 's predefined macro
From: Edward Diener (eldlistmailingz_at_[hidden])
Date: 2020-03-22 09:03:32


On 3/22/2020 9:38 AM, Ville Voutilainen via Std-Discussion wrote:
> On Sun, 22 Mar 2020 at 14:32, Edward Diener via Std-Discussion
> <std-discussion_at_[hidden]> wrote:
>>>> But if a compiler has a documented predefined macro which is not in the
>>>> subclause is it valid C++ to undefine that macro, and must a C++
>>>> standard conforming compiler allow it ?
>>>
>>> No, and no. Do you have an example of such a macro? Do you think it's
>>> valid to name it
>>> in your program, considering http://eel.is/c++draft/lex.name#3 ?
>>
>> I had no intention of naming it in my program, other than undefining it.
>> I actually have a good reason for undefining a compiler predefined macro
>> in the code base in which I am working. However your link suggests that
>> it is not valid C++ to undefine a compiler implemented predefined macro
>> which starts with a double underscore. But I do find the wording of:
>>
>> "In addition, some identifiers are reserved for use by C++
>> implementations and shall not be used otherwise; no diagnostic is required."
>>
>> a bit vague in that "and shall not be used otherwise" is not given any
>> precise meaning. Is undefining a predefined macro "using" the macro ?
>> Probably, but the imprecise meaning of the term "used otherwise" can
>> certainly be improved in that note.
>
> The "shall not be used otherwise" certainly covers any use that would
> change the meaning
> of the token - like undefining it. Or using it as a name in your own
> definition. So, you can actually
> name it, as long as that name means what the implementation defined it
> to mean. Any change
> to that meaning is IFNDR.

What do you mean by "name it" ? In my particular case of a compiler
predefined macro starting with a double underscore you can not do
anything to "name it" that I can see.


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