Re: [boost] Improving the assignment operators of various Boost types

Subject: Re: [boost] Improving the assignment operators of various Boost types
From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-09-10 05:40:50

Thanks for all of your feedback so far.  I just started by submitting
three tickets.  Please double-check:

#2311: any::operator= should have by-value argument
  http://svn.boost.org/trac/boost/ticket/2311
#2312: intrusive_ptr::operator=  should have by-value argument
  http://svn.boost.org/trac/boost/ticket/2312
#2313: multi_index_container::operator=  should have by-value argument
  http://svn.boost.org/trac/boost/ticket/2313

Peter Dimov wrote:
>> /if/ an assignment operator is implemented by means of
>> copy-and-swap, it should preferably be done as follows:
>>
>>  T& operator=(T arg)
>>  {
>>    arg.swap(*this);
>>    return *this;
>>  }
> 
> This is well known (although the typical form is swap(arg)
> instead of arg.swap(*this)).

Is the choice between swap(arg) and arg.swap(*this)) just a matter of
taste?

> It is not widely used in Boost because it creates problems
> with some compilers (I recall Borland being one). The "classic" form is,
> unsurprisingly, much better supported across the board. Things may have
> improved now, of course.

Please let us know if there's a specific compiler version we need to
support that wouldn't allow by-value argument for assignment operators.

David Abrahams wrote:
> In generic code I strongly prefer
> 
>    T& operator=(T arg)
>    {
>      swap(*this, arg);
>      return *this;
>    }

I'm not sure...  It might still compile when T wouldn't have a custom
swap.  If so, it would possibly pick the /generic/ boost::swap or
std::swap, recursively caling the assignment of T and getting into an
infinite loop.  So it seems to me that its more safe to call the swap
/member/ function of T, in this case.

To be continued...

  Niels