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Re: Introducing references

From: Peter C++ <peter.cpp_at_[hidden]>
Date: Thu, 27 Jan 2022 12:42:46 +0100
use boost::optional or tl::optional or jss::object_ptr if you insist of seeing pointer

sent from a mobile device so please excuse strange words due to autocorrection.
Peter Sommerlad
+41-79-432 23 32

> On 27 Jan 2022, at 08:37, Nico Josuttis via SG20 <sg20_at_[hidden]> wrote:
> This was a general comment about pass by pointer.
> However, I wonder :
> Can we really not use std::optional for that?
> I really don't know what effect an optional reference or an optional passed by reference has...
> Am 27. Januar 2022 07:45:55 MEZ schrieb Yongwei Wu <wuyongwei_at_[hidden]>:
>> My point is that using a pointer can be an efficient _implementation_ for ‘passing an optional parameter by reference’. Just that, no more, no less.
>> On Thu, 27 Jan 2022 at 14:15, Nico Josuttis <nico_at_[hidden]> wrote:
>>> yes, there is no real concepts such as pass-by-pointer. I teach that only as workaround for pass-by-reference (especialky used in C).
>>> But this leads to two topics also important to teach :
>>> - pointers versus references as members
>>> - pass by value always decays, PBR does never do that
>>> Am 27. Januar 2022 05:43:57 MEZ schrieb Arthur O'Dwyer via SG20 <sg20_at_[hidden]>:
>>>> On Wed, Jan 26, 2022 at 10:40 PM Yongwei Wu <wuyongwei_at_[hidden]> wrote:
>>>>>> On Thu, 27 Jan 2022 at 04:03, Victor Eijkhout via SG20 <sg20_at_[hidden]> wrote:
>>>>>>> Why do you teach the “pass by pointer”?
>>>>>> My 2 cents here (in addition to Arthur’s good reply). I think there is a use in daily programming to describe ‘potentially null’. Some people prefer std::optional, but it has a bad performance impact, especially on large objects, and it can only be used on an ‘in’ parameter, but not an ‘out’ or ‘in-out’ parameter. If there is no ownership involved and the argument can be ‘missing’, I would recommend using a pointer (as versus a reference that is not allowed to be null).
>>>>> It's certainly important to teach that pointers can be null (and that C++ has a keyword for this — `nullptr`!), but I don't think that's relevant to out-parameters or "pass by pointer" per se. When we're passing by pointer, we're always passing some thing by pointer:
>>>>> f(x, &y); // x is passed by value or const&, we don't care which; y is passed by pointer, indicating an out-parameter
>>>>> In my particular motivating example, where we're just trying to invent a way to pass a std::string efficiently without copying, there's obviously no reason to ever pass a null pointer there; the pointer we pass points to the string, by definition. Likewise for an out-parameter, the pointer we pass points to the place the result is going to go, by definition. Sure, hypothetically someone might call
>>>>> int x;
>>>>> f(x, nullptr);
>>>>> passing garbage for the first parameter and null for the second; but that's obviously foolish and we don't need to go there.
>>>>> (If a student brings it up, there's lots of philosophically interesting stuff around invariants that aren't actually invariant, especially now that we have C++20 Concepts. For example, C++20 defines `std::totally_ordered<float> == true`, despite the existence of `NaN`; with basically the same rationale I gave above: "[comparing things against NaN] is obviously foolish and we don't need to go there." C++ is full of corner cases where something is physically possible but semantically a bad idea. variant::valueless_by_exception() also comes to mind (but I would rather tell a student about NaN than tell them about valueless_by_exception! :D)
>>>>> –Arthur
>>> --
>>> Nico Josuttis
>>> (sent from my mobile phone)
> --
> Nico Josuttis
> (sent from my mobile phone)
> --
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Received on 2022-01-27 11:42:49