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Re: Default move constructor/assignment

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On Thursday, April 5, 2012 1:24:32 AM UTC-5, Juha Nieminen wrote:
> Juha Nieminen <nospam@thanks.invalid> wrote:
> >  I can't find any info about this with google.
> 
>   Seemingly I have encountered such cutting-edge C++11 technology that
> nobody even here seems to know the answers... :)
> 
>   Well, I'll just have to trust that clang++ is correct and
> standard-conforming.


Shalom

The following is from Howard Hinnant in May 2011.
 
The rules for an implicitly generated move constructor are:

If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declared as defaulted if and only if

•X does not have a user-declared copy constructor,
•X does not have a user-declared copy assignment operator,
•X does not have a user-declared move assignment operator,
•X does not have a user-declared destructor, and
•the move constructor would not be implicitly defined as deleted.
The rules for implicitly generated move assignment operators follows the pattern above.

The rules for when a copy constructor is implicitly generated have changed slightly!

If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (8.4). The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.

And similarly for the copy assignment operator:

If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defined as defaulted (8.4). The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor.

Bottom line: The rule of 3 is now the rule of 5. You can either ignore all 5 (if the default behavior works for you), or you need to think about (and likely define) all 5:

1.copy constructor
2.copy assignment
3.move constructor
4.move assignment
5.destructor
 
-----------------------------------------------------------
I'm not sure how up to date this is.  I recall Dave Abrahams
commenting on this subject on cppnext also.

Brian Wood
Ebenezer Enterprises
http://webEbenezer.net

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Thread

Default move constructor/assignment Juha Nieminen <nospam@thanks.invalid> - 2012-04-04 09:21 +0000
  Re: Default move constructor/assignment Juha Nieminen <nospam@thanks.invalid> - 2012-04-05 06:24 +0000
    Re: Default move constructor/assignment Miles Bader <miles@gnu.org> - 2012-04-05 16:48 +0900
    Re: Default move constructor/assignment woodbrian77@gmail.com - 2012-04-05 01:26 -0700
      Re: Default move constructor/assignment Juha Nieminen <nospam@thanks.invalid> - 2012-04-05 09:15 +0000
        Re: Default move constructor/assignment woodbrian77@gmail.com - 2012-04-05 09:35 -0700
          Re: Default move constructor/assignment Juha Nieminen <nospam@thanks.invalid> - 2012-04-06 05:29 +0000
            Re: Default move constructor/assignment woodbrian77@gmail.com - 2012-04-06 23:26 -0700
              Re: Default move constructor/assignment Howard Hinnant <howard.hinnant@gmail.com> - 2012-04-07 10:34 -0700
                Re: Default move constructor/assignment woodbrian77@gmail.com - 2012-04-07 15:59 -0700
                Re: Default move constructor/assignment woodbrian77@gmail.com - 2012-04-07 18:58 -0700
                Re: Default move constructor/assignment SG <s.gesemann@gmail.com> - 2012-04-10 02:21 -0700

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