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

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-03-20 03:35:41

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.

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

 >> 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.

Regards,
Tobias