Re: [Boost-users] [bind][function] Do copy constructors invokedinside boost::bind / boost::function get optimized out?

Subject: Re: [Boost-users] [bind][function] Do copy constructors invokedinside boost::bind / boost::function get optimized out?
From: Steve Lorimer (steve.lorimer_at_[hidden])
Date: 2010-07-27 08:16:25

Thanks Peter

Am I right in saying std::bind for gcc will be based on boost's bind? If
that is the case, then there could be a point?

TIA
Steve

On 27 July 2010 11:41, Peter Dimov <pdimov_at_[hidden]> wrote:

> Steve Lorimer wrote:
>
>> I haven't tested it yet, but I imagine a call to bind with more arguments
>> will involve a lot more copying, as bind looks something like this for
>> argument lists internally:
>>
>> template< class A1 , class A2 , class A3 , class A4  > class list4:
>> private
>> storage4< A1 , A2 , A3 , A4  >
>> template< class A1 , class A2 , class A3 , class A4  > struct  storage4  :
>> public  storage3< A1 , A2 , A3 >
>> template< class A1 , class A2 , class A3  > struct  storage3  : public
>> storage2< A1 , A2  >
>> template< class A1 , class A2  > struct  storage2  : public  storage1< A1
>>  >
>> template< class A1  > struct  storage1
>>
>> Argument A1 is copied for each constructor in the list as it's passed down
>> the inheritance hierarchy.
>>
>
> This is the optimization that makes the placeholders not occupy any
> space... its purpose was to allow function<> to store bind( &X::f, p, _1,
> _2, _3 ) without going to the heap. Unfortunately, you're right that it
> results in a lot more copies. Not sure what can be done here.
>
>
>  Are there plans to implement bind using variadic templates? Not sure if
>> that
>> would reduce the number of copies?
>>
>
> Rvalue references can help, as the copies can be converted into moves. But
> every compiler that has variadic templates and rvalue references also has
> std::bind, so there's not much point.
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