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

Subject: Re: [boost] Improving the assignment operators of various Boosttypes
From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-09-11 07:04:00

>> BTW the swap member function offers a C++03 way to "move assign"
>> [from] an rvalue:
>>
>>  std::vector<int> generate_some_data(void);
>>  std::vector<int> my_data;
>>  // Move the result into my_data:
>>  generate_some_data().swap(my_data);
>>

Giovanni Piero Deretta wrote:
> I guess this is a common idiom, I use it a lot.
>
> OTOH if my_data::operator=() used pass by value (which in the case of
> standard containers is,  unfortunately, not true), the member swap
> wouldn't be necessary.
>
> In practice this means that, for your own classes, or you implement
> the 'smart' operator= or you need the member swap (to swap out of
> temporaries). You do not need both.

When having a by-value argument for operator=, a compiler is /allowed/ 
to do copy elision, but it's not /required/ to do do.  So I'd recommend 
you to just keep swapping your rvalues (using member swap) if you 
already do so anyway  :-)

Kind regards, Niels