220 35153 article Path: news.gmane.org!.POSTED!not-for-mail From: =?UTF-8?Q?Jonathan_M=c3=bcller?= Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Add .emplace() method to std::unique_ptr and std::shared_ptr like it is in std::optional. 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[84.60.195.12]) by smtp.gmail.com with ESMTPSA id d3sm9776402edd.41.2017.10.30.12.07.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 30 Oct 2017 12:07:46 -0700 (PDT) In-Reply-To: <8d668562-154e-4d26-9605-2973bb78f699@isocpp.org> Content-Language: en-US X-Original-Sender: jonathanmueller.dev@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=uhMSFA/V; spf=pass (google.com: domain of jonathanmueller.dev@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=jonathanmueller.dev@gmail.com; dmarc=pass (p=NONE sp=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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:35153 Archived-At: On 30.10.2017 20:01, antanubis@gmail.com wrote: > > The smart pointers and optional behave very differently: optional has > value semantics, so it makes sense to say "create a new object". > I don't think it makes sense to ask the same from unique_ptr. > > In my experience unique_ptr is quite similar to optional. > It is like an optional that doesn't hold the memory while there is no > value in it and that works nicely with forward declarations. But they're not. optional has a copy constructor, deep comparison etc. It just has a similar syntax because (for some reason) it was modelled after a pointer. > > > If we have `emplace()` what about a constructor of the same form? > Yes, there's make_XXX but with class template argument deduction we > don't really want make_XXX anymore. > > As I understand the smart pointer make_* methods won't go anywhere after > the template argument deduction comes in to play because they're not a > helper functions for existing constructors of smart pointers. They can > replace "make_unique(args)" with "unique_ptr(new T(args))" with > returning naked new to the common pattern (which is not looking good) > and they can't replace make_shared at all because of optimized storage > used in make_shared compared to "shared_ptr(new T(args))". > > If we have emplace() in smart pointers perhaps we also would like to > have in_place constructors for smart pointers similar to optional > constructors with std::in_place. Those constructors would improve the > usage of member initialization of smart pointer fields while emplace > improves the usage of assigning of smart pointer variables. This will > allow us to use "A::A() : p(in_place, args)" instead of "A::A() : > p(make_unique(args))" and "p.emplace(args)" instead of "p = > make_unique(args)". > This in place constructor is exactly what I meant there. My comment is meant to be read as "will you also propose an in-place constructor?" > > If `ptr.emplace(obj)` works, what about `ptr = obj`? If you say no to > assignment because it doesn't make sense, why allow emplace? > > object.emplace(value) doesn't always say "assign this value to that > object" even with exactly one argument. For example std::set has > .emplace(value), but it doesn't allow assigning this value to it. > emplace() methods in all standard classes say something like "create a > new object from all those arguments and put it inside" and it works > perfectly for both shared_ptr and unique_ptr. It doesn't mean that but should you also add a corresponding `operator=`? --- I think you are trying to turn smart pointers into indirect, which is already proposed: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0201r1.pdf -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/b59f31eb-2efa-71ed-5b9b-460dcf3741d3%40gmail.com. .