[cfe-dev] Usage of __for_bool_ value in __nat {} classes

[Eric Fiselier via cfe-dev]

On Tue, Jun 5, 2018 at 7:43 PM, Li, Zeyang <a.banknote at gmail.com> wrote:

> @Eric,
>
> Could you elaborate a bit what it means to move the SFINAE into the
> template parameter list? Does it mean the enable_if conversion check could
> be moved to the template parameter list and remove the default __nat
> parameter?
>

Yep, you got it. That's what it entails.


> Does that have any performance implication?
>

You'll be passing one less parameter, so yes, there will be less codegen.
Though I'm not necessarily doing it for that reason.

I personally find using default function arguments for SFINAE to be a worse
solution than default template parameters. Users are able
to accidentally pass "{}" to the function parameter, which should really be
ill-formed. I also find the default template parameter
SFINAE easier to read.

>
> I'm really interested to know as I'm writing my own version of shared_ptr
> without atomics.
> Thanks!
>
> Zeyang
>
> On Wed, Jun 6, 2018 at 7:27 AM, Howard Hinnant <howard.hinnant at gmail.com>
> wrote:
>
>> That sounds right to me.
>>
>> Howard
>>
>> On Jun 5, 2018, at 4:07 PM, Erik Pilkington <erik.pilkington at gmail.com>
>> wrote:
>> >
>> > Looks like the initial purpose was to implement a poor man's explicit
>> operator bool that returned a pointer to member, but it was removed in
>> r151088.
>> >
>> >
>> > On 2018-06-05 3:57 PM, Eric Fiselier via cfe-dev wrote:
>> >> I'm not sure what the purpose is. I agree it doesn't seem to have any
>> effect, and the git blame comes back to the initial commit for libc++.
>> >> Changing the layout of `__nat` now would be an ABI break; so that's
>> out of the question. But I have some upcoming patches that are going
>> >> to remove most usages of it.
>> >>
>> >> @Howard, do you recall what the purpose of this was? Something to do
>> with initializing empty structs maybe?
>> >>
>> >> /Eric
>> >>
>> >> On Tue, Jun 5, 2018 at 2:11 AM, Li, Zeyang via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>> >> I was looking through clang's c++ standard library, and found this
>> class in the shared_ptr class.
>> >>
>> >> class shared_ptr
>> >> ...
>> >> private:
>> >>         struct __nat {int __for_bool_;};
>> >> ...
>> >> };
>> >>
>> >> and I understand that this class is used to detect whether type
>> conversion is possible at compile time, but its member __for_bool_ is never
>> used anywhere in the class or the weak_ptr counterpart. So my question is,
>> what is the point of __for_bool_, why not simply use an empty class for the
>> same purpose?
>> >>
>> >> I'm sure the standard library authors definitely knows better than me.
>> Please help.
>> >>
>> >> Thanks,
>> >> Zeyang
>> >>
>> >>
>> >> _______________________________________________
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>> >> cfe-dev at lists.llvm.org
>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> >>
>> >>
>> >>
>> >>
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>> >
>>
>>
>
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