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Subject: Re: [std-proposals] Fixing some initialization gotchas
From: Maciej Cencora (m.cencora_at_[hidden])
Date: 2019-08-23 09:21:43


Yes, I am proposing to make 'auto x{1};' ill formed because such form is
ambiguous depending on how many elements you pass in braced init list (and
it differs in C++11).

Fixing the fall-out is trivial (either use 'auto x = 1;' or
'std::initializer_list x = { 1 };') and compatible with C++17.

Isn't simplification of initialization rules a good enough argument?

Aah, I forgot to write, I propose to unify direct-init and copy-init as
well:
T a = b;
T a(b);

In C++17 these will do exactly the same thing (minus explicit constructor).

As for the cases where you may get a silent change w.r.t. explicit vs non
explicit constructor, I believe there is only one:
struct A
{
    explicit A(int);
    A(double);
};

A a = 1;
I would be surprised if user actually expected the non-explicit constructor
to be called in such scenario.

For other cases, you will get compiler error do to ambiguity:
struct A
{
    explicit A(short);
    A(double);
};

A a = 1;
Which again is a good thing, because it is not really clear what user meant
in the first place.
And again fixing such fallout is simple (either you use 'A a(short(1));' or
'A a(double(1));') and you still stay compatible with previous C++ versions.

Regards,
Maciej

pt., 23 sie 2019 o 13:49 Timur Doumler <cpp_at_timur.audio> napisał(a):

> IIUC, you are proposing to make existing code like auto x{1}; ill-formed
> that is well-formed today.
>
> Further, you are proposing to silently change the behaviour of existing
> code. Imagine a class with an explicit c’tor and a non explicit one: your
> proposal might silently change which c’tor gets called.
>
> When proposing such breakage, you should really have a good argument why
> that’s worth it, and study how much real-world code would actually be
> affected. How many such initailisations per MLoC exist in existing large
> code bases?
>
> Further, the distinction between direct-list-init and copy-list-init has
> parallels with the much older distinction between direct-init and
> copy-init. You would be breaking that consistency on a conceptual level.
>
> Timur
>
> On 23 Aug 2019, at 11:40, Maciej Cencora via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
> Yes, I have seen it and it was one of motivations to write this proposal.
>
> pt., 23 sie 2019 o 11:15 Eyal Rozenberg <eyalroz_at_[hidden]>
> napisał(a):
>
>> Have you seen Nikolai Josuttis's talk, “The Nightmare of Initialization
>> in C++”
>> https://www.youtube.com/watch?v=7DTlWPgX6zs
>> ?
>>
>> If not, give it a watch; I remember it presents all sorts of eccentric
>> use case and rationales. Josuttis says that he ends up in a state where
>> he is not sure what exactly needs to be "fixed". That would give you
>> perspective on your proposal (and perhaps shoot it down - I'm not sure.)
>>
>> Eyal
>>
>> On 23/08/2019 11:01, Maciej Cencora via Std-Proposals wrote:
>> > If it is already, then yes, and removing the explicit constructor
>> > requirement for copy-list initialization in return statement won't
>> > change that.
>> >
>> > Anyway the main point of this proposal is to unify initialization
>> > behavior a little more in context of variable/member declaration, so
>> > changing explicit constructor requirement in this case could be dropped
>> > if LEWG has strong objections.
>> >
>> > So what do you think?
>> >
>> >
>> >
>> >
>> >
>> > pt., 23 sie 2019 o 01:05 Tony V E <tvaneerd_at_[hidden]
>> > <mailto:tvaneerd_at_[hidden]>> napisał(a):
>> >
>> > I think there was a convincing examples from Howard Hinnant like
>> >
>> > chrono::seconds f()
>> > {
>> > Long();
>> > Function();
>> > ...
>> > if (condition)
>> > return i;
>> >
>> > Morestuff();
>> > Etc();
>> >
>> > return chrono::minutes(j);
>> > }
>> >
>> > Is that function correct?
>> >
>> > Sent from my BlackBerry portable Babbage Device
>> > *From: *Maciej Cencora
>> > *Sent: *Thursday, August 22, 2019 4:56 PM
>> > *To: *Tony V E
>> > *Cc: *sotrdg sotrdg via Std-Proposals
>> > *Subject: *Re: [std-proposals] Fixing some initialization gotchas
>> >
>> >
>> > And what were LEWG arguments for saying no here?
>> >
>> > czw., 22 sie 2019 o 22:55 Tony V E <tvaneerd_at_[hidden]
>> > <mailto:tvaneerd_at_[hidden]>> napisał(a):
>> >
>> >
>> >
>> > On Thu, Aug 22, 2019 at 4:46 PM Maciej Cencora via Std-Proposals
>> > <std-proposals_at_[hidden]
>> > <mailto:std-proposals_at_[hidden]>> wrote:
>> >
>> > Yes.
>> >
>> > After all you are explicit about the return type of the
>> > function (you specified it in function definition), so why
>> > would you not want this to work? There is no possibility for
>> > amibiguity here.
>> >
>> > czw., 22 sie 2019 o 22:36 sdkrystian via Std-Proposals
>> > <std-proposals_at_[hidden]
>> > <mailto:std-proposals_at_[hidden]>> napisał(a):
>> >
>> > So you propose that this should be well formed?
>> >
>> > struct S { explicit operator int() { return 42; } };
>> >
>> > int f()
>> > {
>> > return { S() };
>> > }
>> >
>> >
>> > Having explicit work here has been voted on by the committee in
>> > the past, and LEWG strongly said No.
>> >
>> >
>> > --
>> > Be seeing you,
>> > Tony
>> >
>> >
>> >
>>
> --
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