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

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-03-26 10:46:33

Eric Niebler wrote:
> Frankly, I think the proposed design (function object passed by value, 
> or by reference with explicitly specified template arg) is ugly. It'll
> be ugly both from the user's perspective and in the implementation. 

OK, but why?

For the fusion/functional module the implementation part turned to be
rather smooth.

I must admit that for algorithms (where the function isn't the first
parameter) the different ordering of the template parameters slightly
bugs me.

> If by-value is a perf-win in the common case (perf numbers?), 

I don't think it is. There are cases where it gets optimized better,
however.

You can observe the effect by

     s/F const &/F/g in libs/fusion/example/performance/functional.cpp

using GCC, bjam release-mode options.

Note there is only a small (but clearly measurable) difference in this
case (it's not the one I've been talking about in my previous post --
too much work to reconstruct now, sorry). But, as said, numbers don't 
say much, since they entirely depend on the code that is to be optimized.

> then I'd 
> prefer by-ref semantics to look like this:
> 
>    for_each( seq, ref(func) );

If ef wrap/unwrap optimizes out entirely, this variant looks like a nice 
alternative.

Another one would be to forget about the whole thing. There is a
'mutable' keyword, after all -- and, alternatively, a Function Object 
may reference an external state.

Regards,
Tobias