220 18346 article Path: news.gmane.org!not-for-mail From: Matthew Fioravante Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Use cases for dyn_array Date: Mon, 1 Jun 2015 16:34:32 -0700 (PDT) Lines: 235 Approved: news@gmane.org Message-ID: References: <064e1cec-7a5b-4550-b694-6570e1702776@isocpp.org> <7AC283E4-961E-4AC3-8EDF-AB274DC02BCB@mac.com> <955aafda-3636-4881-9e83-ff07240dd0d7@isocpp.org> <0af21d5d-e95e-41c9-8963-69be0cfbf366@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_4217_177766264.1433201672247" X-Trace: ger.gmane.org 1433201676 17046 80.91.229.3 (1 Jun 2015 23:34:36 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 1 Jun 2015 23:34:36 +0000 (UTC) Cc: potswa@mac.com To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDELF54RTIGRBCGYWOVQKGQE4DTU73A@isocpp.org Tue Jun 02 01:34:36 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f70.google.com ([209.85.218.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YzZE2-000476-IV for gclcip-std-proposals@m.gmane.org; Tue, 02 Jun 2015 01:34:34 +0200 Original-Received: by oihb142 with SMTP id b142sf239706113oih.2 for ; Mon, 01 Jun 2015 16:34:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:content-type:x-original-sender:reply-to:precedence :mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=20Zk4xKTtFe9Bo7RhMQA/1e4bl+Fuie4+wN/McJ0048=; b=Y0BoZ1HHIHEbqlact5vQfCzAYgeA6/DZ00yUc53SJz1UpwdqsPVWKKL+U/h3W22Uj2 WFp4DK3Jau2mM528Ro2CzccQ70PT0YAajvGG1clzC4ZfQ+Hd8tt3MS7yccm38HbKHk9s nrbxEZV1xdquRS2dKVKmcQJYwVWHfmN+1NkmXqTcT55Rg9S1nKOj6acQcFExfFEnwucG qETFFijQ7iKBBoqspdJdhxrpRL3jNvLutahpCsX8+Njs7WOeYewKwu/LcCly0VylLjr+ 5jHa63sotO8d/dlgbhbQcd5baPbcl/xtZA/2igaOx6yBtZ11LiJAE4ZgJ7Txesh34NtO 4HdQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:content-type:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=20Zk4xKTtFe9Bo7RhMQA/1e4bl+Fuie4+wN/McJ0048=; b=YJvycy7QIqtFW4d4aTfYPYstnzVUdZa4zGJlMgvXdsvCNY4FMKxXHorLS3yhOGCyfe RXNgz4Z9/dQRS735j/p6xdQzNjjEmrDvKJzdI9SclPC1qN4vUd0zzmk6Ajge8NrLWr3O G9Ga2dwQhP+fnwwOBmY0zuNqnpeDyM/wFSvy/Rfv513/7Cjo/WWktHvZSiuuwuGWmj/Q NVIb4fYDI9fOF6tW7PS3IBTO2dMIdaYWuzIpKfn4hT868n2q5vbBxK/Zlnw8a28bxxqp pPh3T/AnEPRr8aHZD4oLgDlTsQ+YdWSd0u5oi/68v4BBqH05HJtNqsusEXlrxQfS7j4J WbYA== X-Gm-Message-State: ALoCoQlgpFqeBZePZUDV2taCwb+Oe8KOkB3RXGMnIJMzkZ2BR8keEvIjTQwX2UyN1HLDB7bu/SL4 X-Received: by 10.42.153.202 with SMTP id n10mr40446905icw.21.1433201673519; Mon, 01 Jun 2015 16:34:33 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.41.169 with SMTP id z38ls2811756qgz.77.gmail; Mon, 01 Jun 2015 16:34:32 -0700 (PDT) X-Received: by 10.140.48.11 with SMTP id n11mr287051qga.35.1433201672815; Mon, 01 Jun 2015 16:34:32 -0700 (PDT) In-Reply-To: <0af21d5d-e95e-41c9-8963-69be0cfbf366@isocpp.org> X-Original-Sender: fmatthew5876@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:18346 Archived-At: ------=_Part_4217_177766264.1433201672247 Content-Type: multipart/alternative; boundary="----=_Part_4218_567846071.1433201672247" ------=_Part_4218_567846071.1433201672247 Content-Type: text/plain; charset=UTF-8 Just to make things more clear, here is a possible constructor format for in-place construction of move-only types. This kind of constructor could be used for any type, not just a hypothetical fixed_array. //Class interface template > class fixed_array { public: template fixed_array(size_t n, F&& f, Iterators... iters); //Normal stuff like ::data(), operator[](int), etc... private: T* _begin; T* _end; }; //Inplace constructor implementation template template fixed_array::fixed_array(size_t n, F&& f, Iterators... iters) { _begin = this->allocate(n); _end = _begin + n; struct Construct { Construct(A* alloc, T* p) : _alloc(alloc), _ptr(p) {} template T& operator()(Args&&... args) { _alloc->construct(_ptr, std::forward(args)); return *_ptr; } private: A* _alloc; T* _ptr; }; //exception safety left out for clarify for(size_t i = 0; i < n; ++i) { f(Construct(this, _begin + i), std::forward(iterators)...); advance_all_iterators(iterators...); } } //Usage examples struct Foo { Foo(); Foo(int, double); Foo(string); Foo(const Foo&) = delete; Foo& operator=(const Foo&) = delete; }; auto ci = 0; auto di = 0; std::vector iv = { 1, 2, 3 }; std::vector dv = { 1.0, 2.0, 3.0 }; //Default construct 5 foos auto fa0 = fixed_array(5); //Construct 5 foos using constants auto fa1 = fixed_array(5, [&](auto make) { make(ci, di); }); //Construct 3 foos using a constant and the elements of another container auto fa2 = fixed_array(3, [&](auto make, auto iiter) { make(*iiter, di); }, iv); //Construct 3 foos using the elements of 2 different containers auto fa3 = fixed_array(3, [](auto make, auto iiter, auto diter) { make(*iiter, *diter); }, iv, dv); //Use different constructors auto fa4 = fixed_array(3, [](auto make, auto iiter, auto diter) { if(*iiter < 2) { make(*iiter, *diter); } else { make("Hello"); }, iv, dv); It seems like something like this wouldn't work for aggregates like std::array. I guess the only solution there would be to allow real copy/move elision for aggregate initialization. -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. ------=_Part_4218_567846071.1433201672247 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Just to make things more clear, here is a possible constru= ctor format for in-place construction of move-only types. This kind of cons= tructor could be used for any type, not just a hypothetical fixed_array.
//Class= interface
template <typ= ename T, typename A = =3D std::allocator<T>>
class fixed_array {
  public:
&nbs= p;   = template <= /span><= typename F, typename... Iterators>
    fixed_array(size_t n, F&= ;& f, Iterators... iters);

    //Normal stuff like ::data(), opera= tor[](int), etc...
  private:    T* _b= egin;
  &nbs= p; T* _end;
};

//Inplace constructor implementati= on
temp= late <typename T,typename A> template <typename F, typename... Iterators>
f= ixed_array= <T,A>::= fixed_array(size_= t n, F&& f, Iterators... iters) {
   _begin =3D this-&= gt;allocate(n);
   _end =3D _begin + n;

   st= ruct Construct {
     Construct(A* alloc, T* p) : _a= lloc(alloc), _ptr(p) {}
     template <typename... Arg= s>
     T& operator()(Args&&... args) {       _alloc->construct(_ptr, std::forw= ard<Args>(args));
       return *_pt= r;
     }
    private:
  &= nbsp;  A* _alloc;
     T* _ptr;
  = };

   //exception safety left out for clarify
 &n= bsp; for(size_t i =3D 0; i < n; ++i) {
     = f(Construct(this, _begin + i), std::forward<Iterators>(iterators)...= );
      advance_all_iterators(iterators...);   }
}

//Usage examples
struct Foo {
  Fo= o();
  Foo(int, double);
  Foo(string);
  Foo(cons= t Foo&) =3D delete;
  Foo& operator=3D(const Foo&) =3D= delete;
};

auto ci =3D 0;
auto di =3D 0;
std::vector<i= nt> iv =3D { 1, 2, 3 };
std::vector<double> dv =3D { 1.0, 2.0, = 3.0 };

//Default construct 5 foos
auto fa0 =3D fixed_array<Foo= >(5);

//Construct 5 foos using constants
auto fa1 =3D fixed_ar= ray<Foo>(5, [&](auto make) { make(ci, di); });

//Construct= 3 foos using a constant and the elements of another container
auto fa2 = =3D fixed_array<Foo>(3, [&](auto make, auto iiter) { make(*iiter,= di); }, iv);

//Construct 3 foos using the elements of 2 different c= ontainers
auto fa3 =3D fixed_array<Foo>(3, [](auto make, auto iite= r, auto diter) { make(*iiter, *diter); }, iv, dv);

//Use different c= onstructors
auto fa4 =3D fixed_array<Foo>(3, [](auto make, auto ii= ter, auto diter) { if(*iiter < 2) { make(*iiter, *diter); } else { make(= "Hello"); }, iv, dv);


It seems like so= mething like this wouldn't work for aggregates like std::array. I guess the= only solution there would be to allow real copy/move elision for aggregate= initialization.

--

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