Re: [boost] Improving the assignment operators of various Boosttypes

Subject: Re: [boost] Improving the assignment operators of various Boosttypes
From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-09-10 07:41:09

Niels Dekker:

>>>  T& operator=(T arg)
>>>  {
>>>    arg.swap(*this);
>>>    return *this;
>>>  }

...

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

Yes, I think so. I was just saying that

T& operator=( T arg )
{
    swap( arg );
    return *this;
}

is the form I've seen used (by Dave Abrahams, I think*). It certainly looks 
more "idiomatic" (in the sense of being recognizable as an idiom as opposed 
to ordinary code) because of the unusual swap call.

*) Turns out that I was right in thinking that. Dave's associative_vector 
submission:

http://lists.boost.org/Archives/boost/2000/05/3200.php

contains:

    Self& operator=(Self rhs) { swap(rhs); return *this; } // strong 
guarantee

I was able to find a post by Valentin Bonnard also suggesting the same:

http://tech.groups.yahoo.com/group/boost/message/5001

Incidentally:

http://aspn.activestate.com/ASPN/Mail/Message/boost/1140338

"It doesn't compile with Borland C++Builder 4 or BCC 5.5.

...

The next problem was associative_vector's assignment operator.
Borland didn't recognise it as one and generated a default.
Then it couldn't disambiguate the generated assignment operator
and the one supplied. I changed this to

Self& operator=(const Self &rhs)
{ Self temp(rhs); swap(temp); return *this; }"

My memory is correct on this one, too. I'm not sure why BCC 5.5.1 didn't 
exhibit the problem in my tests.