Re: [boost] [GSoC][Phoenix3] Regarding mutability of arguments passed to phoenix expressions

Subject: Re: [boost] [GSoC][Phoenix3] Regarding mutability of arguments passed to phoenix expressions
From: Eric Niebler (eric_at_[hidden])
Date: 2010-06-11 14:12:15

On 6/11/2010 1:24 PM, Giovanni Piero Deretta wrote:
> On Fri, Jun 11, 2010 at 6:03 PM, Eric Niebler <eric_at_[hidden]> wrote:
>> Well, there's a hitch.
>>
>>  ref[ bigobj += _1 ]
>>
>> Users might reasonably expect this to not make a copy of bigobj, and the
>> way to avoid it is not obvious:
>>
>>  ref[ ref(bigobj) += _1 ]
>>
>> Clearly the user has already stated their intention to capture bigobj by
>> reference and shouldn't have to say it again.
>>
>> I think default capture modes may be unworkable if we want the default
>> mode to be by-value.
>>
> 
> unless you forbid 'naked' lambdas:
> 
>  int x = 0;
>  auto f0 = lambda    [ x += _1 ]; // capture x by val
>  auto f1 = lambda_r [ x += _1 ]; // capture x by ref
>  auto f2 =  x += _1 ; // just a proto expression tree, not a lambda
> 
>  f0(1); //ok
>  f2(1); //ok
>  f3(1); // no operator()
> 
> The lambda syntax becomes more heavy weight, but lambdas do stand up
> more in code. You would have to protect nested lambdas anyway ...

That's true, and I /personally/ wouldn't be opposed to that; I just
don't have a strong preference. This should be Joel's call.

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