220 38947 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allow forward class declarations to match a type alias. Date: Sat, 30 Jun 2018 15:04:00 -0700 (PDT) Lines: 158 Approved: news@gmane.org Message-ID: References: <49cde8d1-312f-4c4c-a26e-91bbb1c56a2d@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_25301_676648564.1530396240632" X-Trace: blaine.gmane.org 1530396116 26246 195.159.176.226 (30 Jun 2018 22:01:56 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 30 Jun 2018 22:01:56 +0000 (UTC) Cc: mihailnajdenov@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBUP437MQKGQEQXQODTA@isocpp.org Sun Jul 01 00:01:51 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f199.google.com ([209.85.213.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fZNwF-0006jJ-1s for gclcip-std-proposals@m.gmane.org; Sun, 01 Jul 2018 00:01:51 +0200 Original-Received: by mail-yb0-f199.google.com with SMTP id q12-v6sf7978965ybp.11 for ; Sat, 30 Jun 2018 15:04:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=WQhulF2M7aehQERR/0GyWf56BmfIccs4/YDLqEY3GfM=; b=vWLZIdcqr3zFsXuUmbOh+cuUEPzK6KqYqE9inO+/2IfdPoT1JcPldfo5NBX9TLFI1p mnG+5P3e0QCmfaaGtawVI9p5YoTvZ0p665WLUxQuZEKAb94bUtoT2VR9gGj++AFAOa+7 ZwLB3BcTF5muWk0ja48MUS1KAvay5+i1foYRRqFiKcSC8vrqqZCfgIaANKBt/mELOQCH FUpdMsGcxIzsPBQVXRXDz1eacu6T9/UEn2WHTLiZ9vCYLXYKxAEVCl4SI/gaQvv4uVvl aHk3wZxRJjuu2aHUEfPfu+cy9G/+/HNUjmYeoOvZFOucUprBfXwx3thM9wuiZOPFST7S WeVA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=WQhulF2M7aehQERR/0GyWf56BmfIccs4/YDLqEY3GfM=; b=dj364jZK11AUiKyyUxyepAPnifsxYL/FKAUo+J4O5nG4TyWsqmzu1eQxdyz0dCaGDh MjIbeuXJw6qeViKDxOhA6PrMr62nDwPqnjHmVr5WMkDaG1EhgBIoIo/tsMuvRxDHDX/R dqm8+ekx0mLTEby5TPKI0ry6uh4mJiyDM0NkgqoklwGBYnrXKtJhon9TtQ/cvdWDqID1 0+Et12IOxjNEkiyE3slQ8sfDyP9BvXuPnsgb9AvHmAmm+dYRa41JGuWvhVzOu7+DddvT myTnQBCe9kYBIbLIvzDpTuErM9TpQMHsSmnWJs311z7x0dTbIs4Xlr+1O6wkHmrGOTy2 svDg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:x-original-sender:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=WQhulF2M7aehQERR/0GyWf56BmfIccs4/YDLqEY3GfM=; b=ZlRZJti1HPeWVTCmo+IaZd+Q5uuqyV7Ba99rJzCeBGx1IEWhmts40JOGVv6htWL9IE 2p2BYcR92N/+5SWH+e59OiY397PIedjD8O3AhjhXQiVuunZRz8I4Uau24hRxfQgQ1WyS 0N7+/aLPxn24tmFollbjpixqvBSyyRzvzQoBQNXcvPG18wpkf+WEDdwFY3xwrx2h4M6y ENQLSGOXoafz2Qnlf2keltlWyxg/lA44LDQY0737ez7H+CiaAshpmKOFtYxDkGsZuioj 8v2TQMe53FA6GOu8r4Smdd2WYRIsBoyISyBkFmrgPn1k2NHLQ9CewAVToKzFyFKhWiOX xjew== X-Gm-Message-State: APt69E0Tpo95xuEeutRJ1VbLzaF16+OXH00NilfcO7Q4ejPBwKeYLoQw ej9/OAmYBM4Aw5h1xOsbD99PFg== X-Google-Smtp-Source: AAOMgpfkLXKXkwz21pPoRi7V5GX8PWIyWsbCc/in9ejVB3Ym3sikw5m1AHMhEtvvx3CPzI0fCPkRZA== X-Received: by 2002:a81:4b90:: with SMTP id y138-v6mr6513057ywa.186.1530396242060; Sat, 30 Jun 2018 15:04:02 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a81:a906:: with SMTP id g6-v6ls2307598ywh.22.gmail; Sat, 30 Jun 2018 15:04:01 -0700 (PDT) X-Received: by 2002:a81:9ac4:: with SMTP id r187-v6mr1948719ywg.4.1530396241219; Sat, 30 Jun 2018 15:04:01 -0700 (PDT) In-Reply-To: X-Original-Sender: jmckesson@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:38947 Archived-At: ------=_Part_25301_676648564.1530396240632 Content-Type: multipart/alternative; boundary="----=_Part_25302_7456484.1530396240632" ------=_Part_25302_7456484.1530396240632 Content-Type: text/plain; charset="UTF-8" On Saturday, June 30, 2018 at 4:23:41 PM UTC-4, Bengt Gustafsson wrote: > > In my previous response I acknowedged the explanation that this has to do > with name mangling, where ultimately the class name would be mangled > according to its real identity, i.e. CudaImage and thus that a > forward declaration "class FloatImage;" would be natural to reject. > > However, on second thought, FloatImage just as any class name is not an > entity in itself that is known to the linker. And as you can't call methods > on a class without seeing its full definition there is no risk that > references to erroneously mangled method names sneak in to an object module > compiled only seeing the forward declaration, right? > With only a forward declaration, you cannot call members. Nor can you call non-member functions that take a reference to the forward declared type (because you would need a valid reference to a `FloatImage`). However, you *can* get a pointer to *functions* that take such types: void func(const FloatImage &); using fptr = void(*)(const FloatImage&); fptr f = func; You can then take `f` and pass it to some API which *itself* can get `FloatImage` objects and pass them to the given function pointer. So the compiler needs to know how `f` works on something approaching an ABI level. And that means knowing if `FloatImage` is an actual type or a type alias (and if so, what it is an alias for). Also, of course, you can call functions that take a pointer to `FloatImage` (you can only pass nullptr, of course). Since pointers are themselves typed, and linking needs to tell the difference, ABI name mangling needs to know if `FloatImage` is a type or a type alias. > With this in mind, is there a technical reason that > my initial construct must be forbidden? I don't see that Nicol's > explanation holds "FloatImage turns out eventually to be a type alias and > not a class". But the very reason for having a type alias is to give a > class another name, so why can't it be forward declared under this > alternate name? > No, the reason to have a type alias is to give a *type* another name. `class FloatImage` will always be a *class*. `using FloatImage = ...;` is whatever `...` happens to be. Your initial construct ought to be forbidden because it is incoherent. You declared that there is a class named `FloatImage`. You cannot later change your mind and decide that it is something else. -- 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/fe3a7806-ec9f-4822-bd79-2d6d61b97c43%40isocpp.org. ------=_Part_25302_7456484.1530396240632 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Saturday, June 30, 2018 at 4:23:41 PM UTC-4, Be= ngt Gustafsson wrote:
In my previous response I acknowedged the explanation that this ha= s to do with name mangling, where ultimately the class name would be mangle= d according to its real identity, i.e. CudaImage<float> and thus that= a forward declaration "class FloatImage;" would be natural to re= ject.

However, on second thought, FloatImage just as any class name= is not an entity in itself that is known to the linker. And as you can'= ;t call methods on a class without seeing its full definition there is no r= isk that references to erroneously mangled method names sneak in to an obje= ct module compiled only seeing the forward declaration, right?
<= /blockquote>

With only a forward declaration,= you cannot call members. Nor can you call non-member functions that take a= reference to the forward declared type (because you would need a valid ref= erence to a `FloatImage`). However, you can get a pointer to func= tions that take such types:

void func(const FloatImage &);
usin= g fptr =3D void(*)(const FloatImage&);

fptr f =3D func;

You c= an then take `f` and pass it to some API which itself can get `Float= Image` objects and pass them to the given function pointer. So the compiler= needs to know how `f` works on something approaching an ABI level. And tha= t means knowing if `FloatImage` is an actual type or a type alias (and if s= o, what it is an alias for).

Also, of course, you = can call functions that take a pointer to `FloatImage` (you can only pass n= ullptr, of course). Since pointers are themselves typed, and linking needs = to tell the difference, ABI name mangling needs to know if `FloatImage` is = a type or a type alias.
=C2=A0
With this in mind, is there a tec= hnical reason that=C2=A0
my initial construct must be forbid= den? I don't see that Nicol's explanation holds "FloatImage tu= rns out eventually to be a type alias and not a class". But the very r= eason for having a type alias is to give a class another name, so why can&#= 39;t it be forward declared under this alternate name?

No, the reason to have a type alias is to giv= e a type another name. `class FloatImage` will always be a class<= /i>. `using FloatImage =3D ...;` is whatever `...` happens to be.

Your initial construct ought to be forbidden because it is = incoherent. You declared that there is a class named `FloatImage`. You cann= ot later change your mind and decide that it is something else.

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/fe3a7806-ec9f-4822-bd79-2d6d61b97c43= %40isocpp.org.
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