220 22803 article Path: news.gmane.org!not-for-mail From: Andrew Tomazos Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Designated initializer list status? 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[2a00:1450:400c:c09::230]) by mx.google.com with ESMTPS id bv6si4988164wjc.97.2015.11.21.10.39.44 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 21 Nov 2015 10:39:44 -0800 (PST) Received-SPF: pass (google.com: domain of andrewtomazos@gmail.com designates 2a00:1450:400c:c09::230 as permitted sender) client-ip=2a00:1450:400c:c09::230; Original-Received: by wmww144 with SMTP id w144so53724801wmw.1 for ; Sat, 21 Nov 2015 10:39:43 -0800 (PST) X-Received: by 10.194.200.233 with SMTP id jv9mr25116788wjc.133.1448131183672; Sat, 21 Nov 2015 10:39:43 -0800 (PST) Original-Received: by 10.28.223.69 with HTTP; Sat, 21 Nov 2015 10:39:43 -0800 (PST) In-Reply-To: X-Original-Sender: andrewtomazos@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of andrewtomazos@gmail.com designates 2a00:1450:400c:c09::230 as permitted sender) smtp.mailfrom=andrewtomazos@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:22803 Archived-At: --047d7bb70c7eca835e05251150e5 Content-Type: text/plain; charset=UTF-8 On Sat, Nov 21, 2015 at 7:37 PM, Andrew Tomazos wrote: > On Sat, Nov 21, 2015 at 6:56 PM, Nicol Bolas wrote: > >> Oh, and for someone coming up with such a proposal, don't forget that in >> C++17, types with base classes that have variables will *also* be >> considered aggregates. So you'll need to come up with a way to use them >> with designated initializers too. >> >> Good luck solving *that* one ;) >> > > The simplest way would be to say that the base classes are default > constructed and the designated initializer list can only refer to the most > derived members. > > struct D : B { int x; } > D d = { .x = 42}; // ok, B default constructed > > Even simpler would say that the designated initializer list constructor is > only generated for classes without base classes. > > struct D : B { int x; } > D d = { .x = 42}; // ill-formed > > Another solution would be to allow .BaseType as a key in a designated > initializer list. > > struct B { int y; } > struct D : B { int x; } > D d = { .B = {.y = 43}, .x = 42}; // ok > > Another solution would be to roll up the base object graph into a single > list of members and the designated initializer list addresses only member > subobjects (and not base subobjects). > > struct B { int y; } > struct D : B { int x; } > D d = { .x = 42, .y = 43}; // ok > > The third and forth solutions are forward-compatible with the first and > second solutions. > > >> >>> One of the things I've been thinking about is that we could have a >>> std::designated_initializer_list unit type (no member functions) - and for >>> aggregate class types a constructor taking such a type is generated. A >>> constructor taking such a type could also be explicitly defaulted. We >>> could then have a syntactic construct designated-init-list. >>> >> >> I don't see the purpose of this. Generally speaking, most types that have >> constructors have invariants, so their data members are private, so you >> can't name them. >> > > Noone is suggesting being able to address private members. The reason I > suggest using a type, is for forwarding of designated initializer lists. > To actually consume them (as an endpoint) you need a compiler-generated > constructor - either implicitly or explicitly defaulted. > > eg > > struct S { int x,y; }; > std::vector v; > v.push_back({.x = 42, .y = 43}); > Sorry this is a bogus example. I meant something like: struct S { int x,y; }; std::optional s; s.emplace({.x = 42, .y = 43}); > So what was the point of providing that type with a constructor? >> > > Apart from the above, its also so we can address the designated > initializer list constructor, to explicit default or explicitlty delete it. > > >> And don't forget: the last time we made special rules for how >> braced-init-lists call constructors, we got screwed with the >> `vector/initializer_list` problem. We shouldn't be making more >> rules that break uniformity farther than it already has been. >> >> I'm not sure what you mean by the "`vector/initializer_list` > problem". > > -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. 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On Sat, Nov 21, 2015 at 7:37 PM, Andrew Tomazos <<= a href=3D"mailto:andrewtomazos@gmail.com" target=3D"_blank">andrewtomazos@g= mail.com> wrote:
On Sat, N= ov 21, 2015 at 6:56 PM, Nicol Bolas <jmckesson@gmail.com> = wrote:
Oh, and for someone coming up with such a p= roposal, don't forget that in C++17, types with base classes that have = variables will also be considered aggregates. So you'll need to = come up with a way to use them with designated initializers too.

Goo= d luck solving that one ;)

The simplest way would be to say that the base classes are default = constructed and the designated initializer list can only refer to the most = derived members.

struct D : B { int x; }
D d =3D { .x =3D 42}; // ok, B default constructed

Even simpler would say that the designated initializer list constructor i= s only generated for classes without base classes.

struct D : B { int x; }
D d =3D { .x =3D 42}; // ill-formed=

Another solution would be to allow .BaseTyp= e as a key in a designated initializer list.

struc= t B { int y; }
struct D : B { int x; }
D d = =3D { .B =3D {.y =3D 43}, .x =3D 42}; // ok

Another solution would be to roll up the base object graph into a sin= gle list of members and the designated initializer list addresses only memb= er subobjects (and not base subobjects).

struct B = { int y; }
struct D : B { int x; }
D d =3D { .x =3D 42,= .y =3D 43}; // ok
=C2=A0
The third and forth solutions= are forward-compatible with the first and second solutions.

=C2=A0
One of the things I've been thinking about i= s that we could have a std::designated_initializer_list unit type (no membe= r functions) - and for aggregate class types a constructor taking such a ty= pe is generated.=C2=A0 A constructor taking such a type could also be expli= citly defaulted.=C2=A0 We could then have a syntactic construct designated-= init-list.

I don't see the purp= ose of this. Generally speaking, most types that have constructors have inv= ariants, so their data members are private, so you can't name them.
=

Noone is suggesting being abl= e to address private members.=C2=A0 The reason I suggest using a type, is f= or forwarding of designated initializer lists.=C2=A0 To actually consume th= em (as an endpoint) you need a compiler-generated constructor - either impl= icitly or explicitly defaulted.

eg

<= /div>
struct S { int x,y; };
std::vector<S> v;
v.push_back({.x =3D 42, .y =3D 43});
=

Sorry this is a bogus example.=C2=A0 I meant something = like:

struct S { int x,y; };
std::o= ptional<S> s;
s.emplace({.x =3D 42, .y =3D 43});


=C2=A0
So what was the point of providing t= hat type with a constructor?

Apart from the above, its also so we can address the designated initiali= zer list constructor, to explicit default or explicitlty delete it.
=C2=A0
And don't forget:= the last time we made special rules for how braced-init-lists call constru= ctors, we got screwed with the `vector<int>/initializer_list<int&g= t;` problem. We shouldn't be making more rules that break uniformity fa= rther than it already has been.

I'm not sure what you mean= by the =C2=A0"`vector<int>/initializer_list<int>` problem= ".


--

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