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Re: Introducing references

From: Nico Josuttis <nico_at_[hidden]>
Date: Thu, 27 Jan 2022 07:14:26 +0100
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)

Nico Josuttis
(sent from my mobile phone) 

Received on 2022-01-27 06:15:21