Re: [Boost-users] [MPL] Runtime traversal of a type sequence untilacondition is true

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2007-05-23 18:39:37

Scott Meyers wrote:
> Daniel Wallin wrote:
>> This is unnecessarily inefficient. It's pretty easy to just use
>> iterators, something like:
>>
>>   template <class First, class Last, class F>
>>   bool do_until_impl(First, Last, F f)
>>   {
>>       boost::value_initialized<typename mpl::deref<First>::type> x;
> 
> Does this not require that mpl::deref<First>::type be default-constructable? 
> Presumably this is the first type in my type sequence, and since I'm 
> dynamic_casting to all types in the sequence, it's not unreasonable to assume 
> that the type I'm casting to may be abstract.  Will this work if my sequence 
> consists of abstract types?  And even if the type is concrete, I can't, in 
> general, know anything about constructing it, i.e., I can't know if it can be 
> value-constructed.

I'm certainly not an MPL expert, but as far as I understand, the same 
problem exists in mpl::for_each. The unary functor passed to 
mpl::for_each looks like:

struct F
{
     template <typename T>
     void operator()(T)
     {
         ....
     }
};

mpl::for_each<seq>(F());

so T is deduced by the argument being passed. T is constructed (probably 
default constructed) and then sent to the functor. Since we usually 
don't want to instantiate the types in the MPL sequence, we use a 
transform meta-function. Something like:

struct F
{
     template <typename T>
     void operator()(mpl::identity<T>)
     {
         ....
     }
};

mpl::for_each<seq, mpl::make_identity<_> >(F());

and then the T's being instantiated are not the types in the sequence, 
but mpl::identity<> of the types in the sequence, which is much better.

The solution proposed here could probably be extended to support 
transform meta-functions just like mpl::for_each does, and the problem 
is solved. I sure hope this (or similar) mpl::for_each_until will be 
added to MPL. I also find it useful.

As I said, I'm not an expert, so I hope I'm not misleading anyone here...