Re: [Boost-users] [fusion] Stateful unary functor for for_each loop

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-03-20 08:02:18

Tobias Schwinger wrote:
> Joel de Guzman wrote:
>> Eric Niebler wrote:
>>> Tobias Schwinger wrote:
>>>> Christian Henning wrote:
>>>>> Hi there, is it possible to have a stateful functor object when using
>>>>> fusion::for_each() loop.
>>>>>
>>>>> I would like to to the following:
>>>>>
>>>> <snip code>
>>>>
>>>> for_each takes the function object by reference to const, so you have to 
>>>> const qualify operator() and make the data members mutable so you can 
>>>> change them anyway.
>>> IMO, this is a bug in Fusion. There should be an overload of for_each 
>>> that takes a function object by non-const reference. 
> 
> For the function invokers I use by-value parameters and the templates 
> are designed to allow explicitly specified template arguments to use 
> references instead. The reason was that by-value arguments can be more 
> efficient for small function objects (tracking side effects) and 
> function pointers.

Really? Do you have some numbers to back that?

> It might be worthwhile to consider this scheme as an alternative to 
> overloading, especially for algorithms which call back multiple times.

Sure. Agreed; also, see below.

>  >> Stateful predicates
>  >> should be supported, and Fusion should be explicit about where and when
>  >> predicates are copied, so that the state doesn't get messed up.
>  >
>> I agree. This is one of the todo items. Noted and added in the queue.
> 
> No doubt here.

Ok, anyway, for the sake of discussion, the stl for_each is:

template <class InputIterator, class UnaryFunction>
UnaryFunction
for_each(InputIterator first, InputIterator last, UnaryFunction f);

Ditto to all the other higher-order algos. What does that tell us?

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net