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

Subject: Re: [boost] Improving the assignment operators of various Boost types
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2008-09-10 14:12:53

Niels Dekker - mail address until 2008-12-31 wrote:
> David Abrahams wrote:
>> In the case of copy-elision for by-value arguments and return values,
>> the compiler is explicitly allowed to _assume_ there is no semantic
>> difference between the original rvalue and its copy.  That's low-hanging
>> fruit for a compiler writer, that pays huge dividends.
> 
> Basically it was concluded that /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;
>   }
> 
> And preferably /not/ as follows:
> 
>   T& operator=(const T & arg)
>   {
>     T(arg).swap(*this);
>     return *this;
>   }

One downside is that self-assignment is made suboptimal. My typical 
operator= implementation is:

   T& operator= (T const& that)
   {
     if (this != &that)
       T(that).swap(*this);
     return *this;
   }

Not sure, however, if it is critical enough.