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

[Erik Pilkington via cfe-dev]

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 <mailto: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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