220 29558 <1a7a1b18-c001-405e-8f7f-09c048c6ee14@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Instantiation of default destructor Date: Mon, 28 Nov 2016 07:11:45 -0800 (PST) Lines: 147 Approved: news@gmane.org Message-ID: <1a7a1b18-c001-405e-8f7f-09c048c6ee14@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2244_1755540733.1480345905535" X-Trace: blaine.gmane.org 1480345909 6383 195.159.176.226 (28 Nov 2016 15:11:49 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 28 Nov 2016 15:11:49 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBMUS6HAQKGQEYIGMUWQ@isocpp.org Mon Nov 28 16:11:44 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f200.google.com ([209.85.223.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cBNap-0000zd-Ev for gclcip-std-proposals@m.gmane.org; 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Mon, 28 Nov 2016 07:11:46 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.27.137 with SMTP id z9ls262558otd.28.gmail; Mon, 28 Nov 2016 07:11:45 -0800 (PST) X-Received: by 10.157.56.132 with SMTP id p4mr567331otc.20.1480345905919; Mon, 28 Nov 2016 07:11:45 -0800 (PST) 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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:29558 Archived-At: ------=_Part_2244_1755540733.1480345905535 Content-Type: multipart/alternative; boundary="----=_Part_2245_1163896384.1480345905535" ------=_Part_2245_1163896384.1480345905535 Content-Type: text/plain; charset=UTF-8 On Monday, November 28, 2016 at 7:14:55 AM UTC-5, Domen Vrankar wrote: > > Hi, > > In this video [1] (time > 57:40) Jason Jurecka was talking about the need to write empty/defaulted > destructor in cpp file if you want to declare a class in header file, use > it in std::unique_ptr and include its declaration only in cpp file: > > //a.h > ... > class T; > > struct A > { > A(T* ptr_); // intentionally before A() constructor - see the question > below > A(); > std::unique_ptr ptr; > }; > ... > > //a.cpp > #include "a.h" > #include "t.h" // declaration of T class > ... > A::A() {} > A::A(T* ptr_) : ptr{ptr_} {} > ... > > This won't work since default destructor is (if I understood the reason > why Tomasz Wota's example in the comment worked: [2] > ) implemented at the > end of the compilation unit. > > For me it's not a big deal to write A::~A() = default; in cpp file but > still since we've had that debate I was wondering if it would be feasible > to change the wording (don't know how much the standard would have to be > changed for that) so that implicit default destructor would be added to the > code at the point of first constructor implementation (in the above example > at the point of A::A(T* ptr_); implementation in .cpp file) and not at the > end of the file where the class was declared? > > You've misunderstood the nature of the problem. The default destructor will call `unique_ptr::~unique_ptr`. And that destructor will call `T::~T()`. The problem is that, unless `T` has been *defined*, you cannot call its destructor. If the compiler defines the destructor for `A`, then it will do so in the class definition. That is, in the header. Which means that anyone who includes this header who doesn't include the definition of `T` beforehand will get a compile error. By declaring the destructor of `A` in the header, then defaulting the destructor in the .cpp, you force the compiler to only generate the class's destructor in the .cpp file. Where you have presumably included the definition of `T`. And thus, people can use `A` without necessarily having the full definition of `T`. -- 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/1a7a1b18-c001-405e-8f7f-09c048c6ee14%40isocpp.org. ------=_Part_2245_1163896384.1480345905535 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Monday, November 28, 2016 at 7:14:55 AM UTC-5, = Domen Vrankar wrote:
Hi,

In this video [1] (time 57:40) Jason Jurecka was t= alking about the need to write empty/defaulted destructor in cpp file if yo= u want to declare a class in header file, use it in std::unique_ptr and inc= lude its declaration only in cpp file:

//a.h
...
class T;

struct A
{
A(T* ptr_);= // intentionally before A() constructor - see the question below
=
A();
std::unique_ptr<T> ptr;
};
...

//a.cpp
#include "a.h"<= br>
#include "t.h" // declaration of T class
<= div>...
A::A() {}
A::A(T* ptr_) : ptr{ptr_} {}<= br>
...

This won't work since default destr= uctor is (if I understood the reason why Tomasz Wota's example in the c= omment worked: [2]) implemented at the end of the compilation unit.
=
For me it's not a big deal to write A::~A() =3D default;= in cpp file but still since we've had that debate I was wondering if i= t would be feasible to change the wording (don't know how much the stan= dard would have to be changed for that) so that implicit default destructor= would be added to the code at the point of first constructor implementatio= n (in the above example at the point of A::A(T* ptr_); implementation in .c= pp file) and not at the end of the file where the class was declared?
=

You've misunderstood the nature of the = problem.

The default destructor will call `unique_ptr::~unique_ptr`.= And that destructor will call `T::~T()`. The problem is that, unless `T` h= as been defined, you cannot call its destructor.

If the compi= ler defines the destructor for `A`, then it will do so in the class definit= ion. That is, in the header. Which means that anyone who includes this head= er who doesn't include the definition of `T` beforehand will get a comp= ile error.

By declaring the destructor of `A` in the header, then de= faulting the destructor in the .cpp, you force the compiler to only generat= e the class's destructor in the .cpp file. Where you have presumably in= cluded the definition of `T`. And thus, people can use `A` without necessar= ily having the full definition of `T`.

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
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/1a7a1b18-c001-405e-8f7f-09c048c6ee14= %40isocpp.org.
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