How to slice with for-range loop ? C++0x

Using range based for loops in C++0X, I know we'll be able to do :

std::vector<int> numbers = generateNumbers();

for( int k : numbers )
{
   processNumber( k );
}

(might be even simpler to write with lambda)

But how should i do if I only want to apply processNumber( k ) to开发者_JAVA技巧 a part of numbers? For example, how should I write this for loop for to apply processNumber() to the half (head or tail) of the numbers? Is "slicing" allowed like in Python or Ruby?

You can use the "sliced" range adaptor from the Boost.Range library:

#include <boost/range/adaptor/sliced.hpp>

using boost::adaptors::sliced;

...

std::vector<int> numbers = generateNumbers();
for( int k : numbers | sliced(0, numbers.size() / 2))
{
     processNumber( k );
}   

One possibility might be boost's iterator_range

(Not having a compiler which supports range-based for, using BOOST_FOREACH instead. I'd expect range-based for work the same, as long as the container or range has the begin and end method.)

#include <boost/foreach.hpp>
#include <boost/range/iterator_range.hpp>
#include <iostream>
#include <vector>

int main()
{
    std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    BOOST_FOREACH(int n, boost::make_iterator_range(v.begin(), v.begin() + v.size() / 2)) {
        std::cout << n << '\n';
    }
}

For convenience you could also make your own slice function, so it would accept indices instead of iterators. Again, it could be based on boost.iterator_range, or not:

#include <cstddef>
#include <iterator>

template <class Iterator>
class iter_pair
{
public:
    typedef Iterator iterator;
    typedef Iterator const_iterator; //BOOST_FOREACH appears to want this
    iter_pair(iterator first, iterator last): first(first), last(last) {}
    iterator begin() const { return first; }
    iterator end() const { return last; }
private:
    iterator first, last;
};

template <class Container>
struct iterator_type
{
    typedef typename Container::iterator type;
};

template <class Container>
struct iterator_type<const Container>
{
    typedef typename Container::const_iterator type;
};

template <class Container>
iter_pair<typename iterator_type<Container>::type>
    slice(Container& c, size_t i_first, size_t i_last)
{
    typedef typename iterator_type<Container>::type iterator;
    iterator first = c.begin();        
    std::advance(first, i_first);
    iterator last = first;
    std::advance(last, i_last - i_first);
    return iter_pair<iterator>(first, last);
}

template <class Container>
iter_pair<typename iterator_type<Container>::type>
    slice(Container& c, size_t i_last)
{
    return slice(c, 0, i_last);
}

//could probably also be overloaded for arrays

#include <cctype>
#include <string>
#include <boost/foreach.hpp>
#include <iostream>

int main()
{
    std::string s("Hello world, la-la-la!");
    BOOST_FOREACH( char& c, slice(s, 2, 11)) {
        if (c == 'l')
            c = std::toupper(c);
    }
    const std::string& r = s;
    BOOST_FOREACH( char c, slice(r, r.size() - 1) ) {
        std::cout << c << " ";
    }
    std::cout << '\n';
}

Generally one would probably be working with iterators in the first place, so it might not be that useful.

Something like this may work (unchecked as I don't have access to a C++0x compiler),

Edit: Checked it on VS10, of course I had to fix numurous errors....

Define a class which is a proxy to any container and whose iterators only return a subset of the container. The example I supply is the simplest one giving the first half but it can be made much more general.

template <class Container>
class head_t {
    Container& c_;
public:
    template <class T>
    class iter {
        T curr_;
        const T& end_;
        int limit_; // count how many items iterated
    public:
        iter(T curr, const T& end) 
            : curr_(curr)
            , end_(end)             
            , limit_(std::distance(curr_, end_)/2)
            { }

        typename Container::value_type operator*() { return *curr_; }

        // Do the equivilant for for operator++(int)
        iter& operator++() {
            if (--limit_ == 0) // finished our slice
                curr_ = end_; 
            else
                ++curr_;
            return *this;
        }

        bool operator!=(const iter& i) const {
            return curr_ != i.curr_; 
        }
    };

    head_t(Container& c) : c_(c) {}
    iter<typename Container::iterator> begin() { 
        return iter<typename Container::iterator>(c_.begin(), c_.end()); 
    }

    iter<typename Container::iterator> end() {
        return iter<typename Container::iterator>(c_.end(), c_.end()); 
    }    
};

template <class T>
head_t<T> head(T& t) { return head_t<T>(t); }

And then you use it in the loop:

for( int k : head(numbers) )