Re: [boost] phoenix::bind

Subject: Re: [boost] phoenix::bind
From: Eric Niebler (eric_at_[hidden])
Date: 2008-09-29 12:35:46

Peter Dimov wrote:
> Eric Niebler:
> 
>> Lambda and phoenix differ as to whether local variables are captured 
>> by value or by reference (phoenix always captures by value, lambda 
>> tries to guess what you intend based on usage).
> 
> Are you sure that
> 
>    int v[] = { 1, 2, 3 };
>    std::for_each( v, v + 3, std::cout << arg1 << std::endl );
> 
> captures std::cout by value? :-)

OK, OK, you got me. Even for Phoenix, the rules are not simple. Phoenix 
captures by value *when it can* (except for arrays, which are by 
reference, I think), or when the user specifies by-ref with 
phoenix::ref. But the decision is based on object's type, not on how the 
object is used in the expression, as it is with boost.lambda. Consider 
that with boost.lambda:

   int ii = 0;
   ii += _1;   // ii captured by reference here ...
   _1 += ii;   // ... but ii captured by value here

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com