Re: [Boost-users] [boost] [review][constrained_value] compile-time lambdas

Subject: Re: [Boost-users] [boost] [review][constrained_value] compile-time lambdas
From: Gordon Woodhull (gordon_at_[hidden])
Date: 2008-12-11 20:00:13

>> I guess we have to wait for decltype to be able to use lambda
>> expressions as predicates here?
>
> Why? There are already examples with lambda predicates in the docs.

Yes, constrained_value does support lambdas at runtime using runtime  
boost::function.

I was hoping that we will also be possible to specify lambdas at  
compile time, which is more efficient because boost::function incurs  
cost equivalent to a virtual method call (it prohibits inlining and  
dereferences a pointer).  And there is the cost of storing the  
function pointer too...

I am imagining things like constrained_value<int, decltype(_1>0 && ! 
bind(isnan,_1))>.

In my perfect imaginary world this would be a zero-size predicate  
which is default constructable because it carries no data.  Thus the  
expression doesn't need to be duplicated on construction and is stored  
entirely in the constrained_value typedef.

0 might have to be construct<mpl::integral_c<0> >()    :-D

I'm sure there are 1000 things wrong with this, just fantasizing...  I  
guess this also falls into the bin of "what's the perfect predicate  
library?"