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Subject: Re: [std-proposals] Proposal - Allow '.' operator to work on pointers (again sortof)
From: J Decker (d3ck0r_at_[hidden])
Date: 2020-03-05 06:35:20


On Thu, Mar 5, 2020 at 3:55 AM Arvid Norberg <arvid.norberg_at_[hidden]>
wrote:

> On Thu, Mar 5, 2020 at 12:29 PM J Decker via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> [...]
>>> Smart pointers were given as an example.
>>>
>>
>> Smart pointers continue to work as they do, because they are an object
>> that contains a pointer to an object, and are not 'pointers'. they are of a
>> class type 'smart pointer', they are not pointers to a class of type 'smart
>> pointer'.
>>
>> I provided examples, in this list and updated in the gist, of how pointer
>> to smart pointers can still work, and actually '.' operator which can
>> perform indirection can simplify that use case by still just being 'access
>> property of pointer'.
>>
>
> Here's an example:
>
> std::shared_ptr<Foo> p = ...;
>

There is no '*' or '&' or [] there. The proposed modification changes
nothing.

> std::cout << p.unique() << '\n';
>
> Does call a function on shared_ptr<> or on Foo?
>

> (hint: unique() used to exist on shared_ptr, was deprecated in C++17 and
> removed in C++20)
>
> If I understand your proposal correctly, it would break the feature of
> this code failing to compile in C++20.
>

again, that is not a *(pointer), that is an instance of a class, aka an
object. '.' remains working on objects exactly the same. and C++20
removing .unique() will fail. there's no pointer to perform indirection
on. It was certainly bad phrasing to say "allow operation '.' to behave
like '->' " .

>
> Perhaps more importantly, if there are ever *new* member functions added
> to shared_ptr<> in a future version of C++, that would case similar kinds
> of breaking of code.
>
and still it's not a pointer to class as the left hand value of '.' .
Smart pointers are instances of pointers... a unique pointer can be a **.
For instance in my C library I pass the address of the pointer of the list,
which allows me to update that in-place; pass by reference even... and
forwarding that ** keeps the original pointer instance all the way back so
there's really only 1 instance of the reference to the list in memory.
(But, I digress, you didn't use any * and I'm spewing them all over).

J

> --
> Arvid Norberg
>



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