AMDG On 08/01/2012 05:34 AM, Joel de Guzman wrote:
3. What is your evaluation of the documentation?
I just skimmed through the docs to get what I need (see below). It would be good to have examples at the bottom of each of the reference sections like that of MPL, Fusion and the PPlib. Example:
http://www.boost.org/doc/libs/1_50_0/libs/mpl/doc/refmanual/fold.html
At the bottom, we have an example.
Noted.
5. Did you try to use the library? With what compiler? Did you have any problems?
Yes. I tried to use it as a replacement for Boost.Function in Boost.Spirit. It worked like a charm. My only difficulty was in trying to figure out how to detect a default-initialized 'any'. With Boost.Function, you can use an instance in a condition, e.g. if (f) ... This seems to be not the case with 'any'. I'm pretty sure this is something simple to do, but I can't dig it out in the docs.
any_cast to void* is the simplest way that exists right now. Something like this would enable a bool conversion: template<class T = _self> struct empty_bool_conversion : mpl::vector<> {}; template<class T, class Base> struct concept_interface<empty_bool_conversion<T>, Base, T> : Base { explicit operator bool() const { return any_cast<void*>(static_cast< const typename derived<Base>::type*>(*this)); } };
I ended up casting to void*. It turns out that it returns 0 (null) if any hasn't been initialized yet. I'm not sure about the cost of that though (would that have to use RTTI? It seems that way as it says in the any_cast docs: http://tinyurl.com/d8sx46m where it says that typeid_ is required.
I'll fix the documentation. RTTI shouldn't be required for a cast to void*.
I did a CT test on one of the largest examples in spirit. I compared it against 1) the original implementation using Boost.Function and 2) A hand-coded implementation using virtual functions. Here are the results:
TypeErasure: 01:14 secs Boost.Function: 00:55 secs Virtual Function: 00:52 secs
That's expected. There are a number of constructs that I know are inefficient in my metaprogramming. I haven't made any attempt to optimize it yet.
For that matter, I'm not ready to use TypeErasure yet for my purpose, but I'm hoping that Steven can improve CT. It might also be possible that I am using TypeErasure incorrectly when I resort to any_cast to test for null any. Would this explain the CT slowdown? I am not sure.
Using any_cast like this should have little effect on compile-time performance.
I'd also suggest the addition of an easy to use Boost.Function replacement in there that can be used out of the box with full compliance.
It isn't possible to replicate Boost.Function perfectly. a) Boost.Function doesn't care about the constness of function call operator. b) Boost.Function has special handling for boost::ref I don't really want to replicate either behavior. In Christ, Steven Watanabe