Re: [Boost-users] [Range] Adapt 2-iterator container constructors for Range?

2008/9/24 Nat Goodspeed

As I work with Boost.Range, I frequently want to call a method returning an iterator_range and, from that range, instantiate a standard container. I have a couple of helpers I'd like to propose for inclusion in the Boost.Range library.

/** * From any range compatible with Boost.Range, return an instance of any * class with a constructor accepting a compatible iterator pair. */ template TYPE make_from_range(const RANGE& range) { return TYPE(boost::begin(range), boost::end(range)); }

It would probably be appropriate to provide a non-const RANGE& overload as well.

make_from_range<CONTAINER>(range) is helpful for, e.g., passing an argument to a function accepting const CONTAINER&. But what if you want to declare a local instance of such a container?

/** * From any range compatible with Boost.Range, instantiate any class * with a constructor accepting a compatible iterator pair. */ template<class TYPE> struct instance_from_range: public TYPE { template<typename RANGE> instance_from_range(const RANGE& range): TYPE(boost::begin(range), boost::end(range)) {} // Again, a non-const RANGE& overload would probably be useful. };

Usage example:

instance_from_range< std::vectorstd::string > my_vector(function_returning_range_of_string());

Why not vector<string> my_vector(make_from_range(get_range()); ? RVO will make sure that no copies of a container are created. If you don't want to specify vector<string> twice you can follow the approach of Boost.Assign -- create an object with a conversion to anything and let it be converted to a container. template <class Range> struct from_range_impl { from_range_impl(const Range& range) : range_(range) {} template <class Container> operator Container() const { return Container(begin(range_), end(range_)); } private: const Range& range_; }; template <class Range> from_range_impl<Range> make_from_range(const Range& range) { return from_range_impl<Range>(range); } Example: void foo(const vector<string>& v); iterator_range range; foo(make_from_range(get_range(range)); vector<string> v(make_from_range(range)); Sometimes you'll still want to explicitly specify type of the container that should be created by make_from_range (for example when you pass it to an overloaded function), but you can keep both versions of make_from_range and use each of them when appropriate. Roman Perepelitsa.