Re: [boost] [review] Fusion review, finalization

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2006-05-10 18:17:34

Tobias Schwinger wrote:
> 
> * Design
> 
> 1. I'm missing an extension point in the generators to turn them into a 
> type-preserving variadic input facility. Elaboration:
> 
> Currently e.g. "make_vector" is an inline function. My suggestion is to turn it into 
> a global constant of the type of a class template instantiation e.g. "vector_generator<F>".
> 
>   vector_generator<identity_function> const make_vector = ...
> 
> This way it's easy to create user-defined function objects, that take a variable number 
> of arguments, turn them into a tuple and finally use this information to create a 
> user-defined object (that is, entirely without using Boost.PP). Of course that beast 
> would be able to take a stateful functor in its ctor.

Hmmm -- I had hoped for a comment on this one... 
Did my review read too negative? Was something unclear?

> 
> * Did you try to use the library? With what compiler? Did you have any problems?
> 
> Yes. 
> 

<snip>

> 
> I'm still playing with it and hope I can add some details here, soon.
> 

I wanted to use Fusion to write something (well, at least half-way) real for review, so I chose to solve this problem:

 Call one of a heterogenous set of functors (their call operators might 
 be templates) based on a runtime index with a complexity of O(log N) 
 (basically what an n-ary tree structure of nested switch/case blocks
 does).

The solution
o handles unary functors only and doesn't bother with the result (simplicity),
o contains numerous workarounds and is far away from being perfect,
o requires an MPL-patch [ http://tinyurl.com/ljfdm] to compile, but
o nevertheless documents my fight with the library, hopefully including lots of mistakes a new user is tempted to make, so it might work good to inspire upcomnig examples.

 http://tinyurl.com/l5bxl  (source file in the vault)

Fusion rocks!

Regards,

Tobias