220 29860 <6911ac18-c144-4e4b-9fad-fe360fbb4c55@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Jared Grubb Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: New smart pointer: CopyConstructible and CopyAssignable flavor of unique_ptr Date: Fri, 16 Dec 2016 18:43:31 -0800 (PST) Lines: 400 Approved: news@gmane.org Message-ID: <6911ac18-c144-4e4b-9fad-fe360fbb4c55@isocpp.org> References: <13702f32-32c2-447f-9996-3624fd678675@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2494_1330130897.1481942611913" X-Trace: blaine.gmane.org 1481942614 28856 195.159.176.226 (17 Dec 2016 02:43:34 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 17 Dec 2016 02:43:34 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDVKDF4E7IDRBVGM2LBAKGQEFPDENRI@isocpp.org Sat Dec 17 03:43:30 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f199.google.com ([209.85.161.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cI4y9-0006a0-HU for gclcip-std-proposals@m.gmane.org; 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Fri, 16 Dec 2016 18:43:33 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.46.13 with SMTP id q13ls9601058otb.20.gmail; Fri, 16 Dec 2016 18:43:32 -0800 (PST) X-Received: by 10.157.11.248 with SMTP id 111mr371714oth.4.1481942612729; Fri, 16 Dec 2016 18:43:32 -0800 (PST) In-Reply-To: <13702f32-32c2-447f-9996-3624fd678675@isocpp.org> X-Original-Sender: jared.grubb@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:29860 Archived-At: ------=_Part_2494_1330130897.1481942611913 Content-Type: multipart/alternative; boundary="----=_Part_2495_93022081.1481942611913" ------=_Part_2495_93022081.1481942611913 Content-Type: text/plain; charset=UTF-8 On Friday, December 16, 2016 at 3:32:25 AM UTC-8, Alexandre Isoard wrote: > > *Proposal* > > template class Cloner = std::default_clone, > class Deleter = std::default_delete > > > class copy_ptr > > std::copy_ptr is a smart pointer that owns and manages another object > through a pointer and disposes of that object when the copy_ptr goes out > of scope. > Additionally, it allocates a clone of the underlying object when it is > copied from, ensuring one copy_ptr manages each allocated object. > > The object is disposed of using the associated deleter when either of the > following happens: > > - the managing copy_ptr object is destroyed > - the managing copy_ptr object is assigned another pointer via > operator= > or > reset() . > > The object is disposed of using a potentially user-supplied deleter by > calling get_deleter()(ptr). The default deleter uses the delete operator, > which destroys the object and deallocates the memory. > > The object is cloned using the associated cloner when either of the > following happens: > > - an other copy_ptr object is copy constructed from the managing > copy_ptr > - an other copy_ptr object is copy assigned from the managing copy_ptr > > The object is cloned using a potentially user-supplied cloner by calling > get_cloner()(ptr). The default cloner uses the new operator and the copy > constructor of the underlying type, which allocate a new object and copy > its original content. > > A copy_ptr may alternatively own no object, in which case it is called > empty. > > The class satisfies the requirements of MoveConstructible > , > MoveAssignable , > CopyConstructible > and > CopyAssignable (the > later two may require T to be CopyConstructible). > > *Member Types* > > - pointer: T* > - element_type: T, the type of the object managed by this copy_ptr > - cloner_type: Cloner, the function object or lvalue reference to > function or to function object, to be called from the copy constructor and > copy assignment operators > - deleter_type: Deleter, the function object or lvalue reference to > function or to function object, to be called from the destructor > > > *Member Functions* > > - operator=: assigns the copy_ptr > - release: returns a pointer to the managed object and releases > ownership > - reset: replaces the managed object > - swap: swaps the managed objects > - get: returns a pointer to the managed object > - get_cloner: returns the cloner that is used for copy the managed > object > - get_deleter: returns the deleter that is used for destruction of the > managed object > - operator bool: checks if there is an associated managed object > - operator*, operator->: dereferences pointer to the managed object > - make_copy: creates a copy pointer that manages a new object > - operator==, operator!=, etc: compares to an other copy_ptr or with > nullptr > > You have a major issue with equality that is fatal to the proposal, IMO. Copying creates a new object. If equality is defined on the pointer, then copies are *never* equal to their original (except nullptr case). If you define equality by the value-type, then you have confusingly different semantics from the other smart-pointer types. You could delete the equality operators (maybe keep "operator==(std::nullptr_t)" only), but that also sounds like a fatal flaw. > > - std::swap: specializes the std::swap algorithm > - std::hash: hash support for std::copy_ptr > > *Example* > > #include > #include > > struct Foo > { > Foo() { std::cout << "Foo::Foo()\n"; } > Foo(const Foo& o) { std::cout << "Foo::Foo(const Foo&)\n"; } > ~Foo() { std::cout << "Foo::~Foo()\n"; } > void bar() { std::cout << "Foo::bar()\n"; } > } > > void f(const Foo&) > { > std::cout << "f(const Foo&)\n"; > } > > int main() > { > std::copy_ptr p1(new Foo); // p1 owns Foo > if (p1) p1->bar(); > > { > std::copy_ptr p2(std::move(p1)); // now p2 owns Foo > f(*p2); > > p1 = p2; // cloned into p1 > std::cout << "destroying p2...\n"; > } > > if (p1) p1->bar(); > > // Second Foo instance is destroyed when p1 goes out of scope > } > > Output: > > Foo::Foo() > Foo::bar() > f(const Foo&) > Foo::Foo(const Foo&) > destroying p2... > Foo::~Foo() > Foo::bar() > Foo::~Foo() > > -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On Friday, December 16, 2016 at 3:32:25 AM UTC-8, = Alexandre Isoard wrote:
Proposal

template<class<= /span> T,
=C2=A0 =C2=A0 class Cloner =3D std::default_clone<T>,
=C2=A0 =C2=A0 = class <= span style=3D"color:#606">Deleter = =3D std:= :default_delete<T>
> class copy_ptr

std::copy_ptr is a= smart pointer that owns and manages another object through a pointer and d= isposes of that object when the copy_= ptr goes out of scope.
Additionally, it allocates a clone of the= underlying object when it is copied from, ensuring one copy_ptr manages each allocated object.

Th= e object is disposed of using the associated deleter when either of the fol= lowing happens:
  • the managing copy_ptr object is destroyed
  • the managing copy_ptr object is assigned another poin= ter via operator=3D or reset().
The object is disposed of using a= potentially user-supplied deleter by calling get_deleter()(ptr). The default deleter uses the delete operator, which destroys the obje= ct and deallocates the memory.

The object is clone= d using the associated cloner when either of the following happens:
  • an other copy_ptr o= bject is copy constructed from the managing copy_ptr
  • an other copy_ptr object is copy assigned fr= om the managing copy_ptr
The object is cloned using a potenti= ally user-supplied cloner by calling = get_cloner()(ptr). The default cloner uses the new operator and the copy constructor of the underl= ying type, which allocate a new object and copy its original content.
=
A copy_ptr may alternative= ly own no object, in which case it is called empty.

The class= satisfies the requirements of MoveConstructible= ,=C2=A0MoveAssignable, CopyAssignable=C2=A0(t= he later two may require T to be CopyConstructible).

Member Types
  • pointer: T*
  • element_type: T, the type of the object managed by thi= s copy_ptr
  • cloner_type:=C2=A0Cloner, the function object or lvalue reference to f= unction or to function object, to be called from the copy constructor and c= opy assignment operators
  • dele= ter_type:=C2=A0Deleter,= the function object or lvalue reference to function or to function object,= to be called from the destructor

Mem= ber Functions
  • operator=3D: assigns the copy_ptr
    • <= li>release: returns a pointer to the managed object and releases ownership<= /li>
  • reset: replaces the managed object
  • swap: swaps the managed = objects
  • get: returns a pointer to the managed object
  • get_cl= oner: returns the cloner that is used for copy the managed object
  • g= et_deleter: returns the deleter that is used for destruction of the managed= object
  • operator bool: checks if there is an associated managed obj= ect
  • operator*, operator->: dereferences pointer to the managed o= bject
  • make_copy: creates a copy pointer that manages a new object
  • operator=3D=3D, operator!=3D, etc: compares to an other copy_ptr or = with nullptr
You have a major issue = with equality that is fatal to the proposal, IMO.

Copying creates a = new object. If equality is defined on the pointer, then copies are never= equal to their original (except nullptr case). If you define equality = by the value-type, then you have confusingly different semantics from the o= ther smart-pointer types.

You could delete the equality operators (m= aybe keep "operator=3D=3D(std::nullptr_t)" only), but that also s= ounds like a fatal flaw.
  • std::swap: specializes the std::swap al= gorithm
  • std::hash<std::copy_ptr>: hash support for std::copy_= ptr
Example

#include<iostream>
#include<memory>= ;

struct Foo
{
=C2=A0 =C2=A0 Foo() =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 { std::cout << "Foo::Foo()\n"; }
=C2=A0 =C2=A0 Foo(const Foo& o) { std= ::cout = << "Foo::Foo(const Foo&)\n"<= /span>; }
=C2=A0 = =C2=A0 ~Foo() = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0= { std::= cout &l= t;< = "Foo::~Foo()\n"; }
=C2=A0 =C2=A0 void bar() =C2=A0 =C2=A0 =C2=A0 =C2=A0{ std::cout << "Foo::bar()\n"; }
}

void= f(const Foo&)
{
=C2=A0 =C2=A0 std::cout << "f(const Foo&)\n";
}=

int main()
{
<= div>=C2=A0 =C2=A0 std::copy_ptr<Foo> p1(new Foo); =C2=A0// p1 owns Fo= o
=C2=A0 =C2=A0 if (p1) p1->bar();
=C2=A0
= =C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 std::copy_ptr<Foo&= gt; p2(std::move(p1)); =C2=A0// now p2 owns Foo
=C2=A0 =C2=A0 =C2= =A0 =C2=A0 f(*p2);
=C2=A0
=C2=A0 =C2=A0 =C2=A0 =C2=A0 p= 1 =3D p2; =C2=A0// cloned into p1
=C2=A0 =C2=A0 =C2=A0 =C2=A0 std= ::cout << "destroying p2...\n";
=C2=A0 =C2=A0 }
=C2=A0
=C2=A0 =C2=A0 if (p1) p1->bar();
=C2= =A0
=C2=A0 =C2=A0 // Second Foo instance is destroyed when p1 goe= s out of scope
}

Output:
=
Foo::Foo()
Foo::bar()
f(const Foo&)
Foo::Foo(const Foo&)
destroying p2...
Foo::~Foo()
Foo::bar()
Foo::~Foo()

--
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/6911ac18-c144-4e4b-9fad-fe360fbb4c55= %40isocpp.org.
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