it-works/ranges/tuple_element.hpp

//
//  main.cpp
//  temp
//
//  Created by Joshua Moerman on 9/3/11.
//  Copyright 2011 Vadovas. All rights reserved.
//

/*
 Some helper function to easily iterate over a container of tuples. In
 particular this is useful with maps. This is meant to be used in
 range-based for loops.
 
 Example:
	map<string, int> m = {...};
	for(auto & str : keys(m))
		cout << str << "\t";
	cout << endl;
	for(auto & i : values(m))
		cout << i << "\t";
 
 keys() let you iterate over the keys of a map (the elements returned are
 const references). values() let you iterate over the values.
 There is a more general function for any container over tuples: nth_values.
 
 Example:
	vector<tuple<int, int, const int>> v = {...};
	for(auto & i : nth_values<2>(v)){
		cout << i << endl;
		++i;		// error: attempting to write to read-only variable
					// that's because i is a const ref.
	}
 
 That's all :). It's very trivial to just use i.first or get<0>(i),
 but this might be very handy when debugging or something.
 
 */

#include <utility>

namespace details {

	/* lazy implementation, relies on inheritance
	 only op* and op-> are overloaded (to return a specific element in
	 the pair or tuple (note that std::get and std::tuple_element
	 are overloaded for pair, to make pair look like a tuple :) ).
	 */
	template <size_t N, typename Iterator>
	struct tuple_iterator : public Iterator {
		tuple_iterator(Iterator it)
		: Iterator(it)
		{}
		
		typename std::tuple_element<N, typename Iterator::value_type>::type &
		operator*(){
			return std::get<N>(*(Iterator&)(*this));
		}
		
		typename std::tuple_element<N, typename Iterator::value_type>::type &
		operator->(){
			return std::get<N>(*(Iterator&)(*this));
		}
	};

	template <size_t N, typename Iterator>
	struct const_tuple_iterator : public Iterator {
		const_tuple_iterator(Iterator it)
		: Iterator(it)
		{}
		
		typename std::tuple_element<N, typename Iterator::value_type>::type const &
		operator*(){
			return std::get<N>(*(Iterator&)(*this));
		}
		
		typename std::tuple_element<N, typename Iterator::value_type>::type const &
		operator->(){
			return std::get<N>(*(Iterator&)(*this));
		}
	};

	/* The const_iterator is not really needed, since no-one will actually
	 store a const tuple_container_wrapper, but it's here for completeness.
	 */
	template <size_t N, typename Container>
	struct tuple_container_wrapper {
		typedef tuple_iterator<N, typename Container::iterator> iterator;
		typedef const_tuple_iterator<N, typename Container::const_iterator> const_iterator;
		
		tuple_container_wrapper(Container & c)
		: c(c)
		{}
		
		iterator begin(){
			return iterator(c.begin());
		}
		
		iterator end(){
			return iterator(c.end());
		}
		
		const_iterator begin() const {
			return const_iterator(c.cbegin());
		}
		
		const_iterator end() const {
			return const_iterator(c.cend());
		}
		
		const_iterator cbegin() const {
			return const_iterator(c.cbegin());
		}
		
		const_iterator cend() const {
			return const_iterator(c.cend());
		}
		
	private:
		Container & c;
	};
	
	template <size_t N, typename Container>
	struct const_tuple_container_wrapper {
		typedef const_tuple_iterator<N, typename Container::const_iterator> const_iterator;
		
		const_tuple_container_wrapper(Container const & c)
		: c(c)
		{}
		
		const_iterator begin() const {
			return const_iterator(c.cbegin());
		}
		
		const_iterator end() const {
			return const_iterator(c.cend());
		}
		
		const_iterator cbegin() const {
			return const_iterator(c.cbegin());
		}
		
		const_iterator cend() const {
			return const_iterator(c.cend());
		}
		
	private:
		Container const & c;
	};
	
}

/* nth_values() is the general function to iterate over a specific
 data element in the container (of tuples/pairs).
 For map-types there are the functions keys() and values(), which are
 special cases of this function.
 */
template <size_t N, typename Container>
details::tuple_container_wrapper<N, Container>
nth_values(Container & m) {
	return details::tuple_container_wrapper<N, Container>(m);
}

template <size_t N, typename Container>
details::const_tuple_container_wrapper<N, Container>
nth_values(Container const & m) {
	return details::const_tuple_container_wrapper<N, Container>(m);
}

// NOTE: keys in a map are always const, so a const-only function suffices
template <typename Map>
details::const_tuple_container_wrapper<0, Map>
keys(Map const & m) {
	return nth_values<0>(m);
}

template <typename Map>
details::tuple_container_wrapper<1, Map>
values(Map & m) {
	return nth_values<1>(m);
}

template <typename Map>
details::const_tuple_container_wrapper<1, Map>
values(Map const & m) {
	return nth_values<1>(m);
}