Re: [Boost-users] metafunction question

Subject: Re: [Boost-users] metafunction question
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2010-05-11 19:04:06

> 
> the return type cannot be parametrized based on the const-ness of
> "this". but parametrizing _functions_ based on constness rarely makes
> sense anyway, the best aproach imho is to keep the overloads but let
> them only handle the const/non-const specific stuff and forward the
> rest.
> in your example:
> 
> private:
>    node_ptr find_node(x) const;
> public:
>    iterator find(x){ return iterator(find_node(x)); }
>    const_iterator find(x) const{ return const_iterator(find_node(x)); }
> 
> 

That was exactly what I showed him.  And that would work for "contained" stuff that was not contained by value.  But, the find_node works using the inherited iterators, not his own mechanisms.  So, the private iterator do_find(x) const finds itself limited to const_iterator in its own implementation from the getgo.

I was thinking

template <typename Collection, typename Iter= get_proper_iter<Collection>::type >
Iter do_find(Collection& c, find_params x);

and the question is, is there already a handy metafunction get_proper_iter<C> that will return C::const_iterator if C is const qualified, and C::iterator otherwise?

I suppose something like
   mpl::if_<std::tr1::is_const<C>, typename C::const_iterator,  typename C::iterator>
wrapped up as the interesting part of the metafunction.  I've never tried anything like that, but I just read the first part of "C++ Template Metaprogramming" book and I see if_ used on page 63.

--John



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