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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

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