namespace boost {

template<class Cases, class Int, class F>
typename F::result_type switch_(Int n, F f);

template<class Cases, class Int, class F, class Default>
typename F::result_type switch_(Int n, F f, Default default_);

}

Header

#include <boost/switch.hpp>

Generates a switch statement. If the value of one of the elements of Cases is equal to n, then this function will call f with a parameter of that type. Otherwise it will pass n to default_ or throw an exception depending on whether default_ was specified. For example

template<class FusionSequence>
struct print_nth_function {
    typedef void result_type;
    template<class Case>
    void operator()(Case) const {
        std::cout << fusion::at<Case>(sequence) << std::endl;
    }
    print_nth(const FusionSequence& s) : sequence(s) {}
    const FusionSequence& sequence;
};

struct throw_out_of_range {
    template<class Int>
    void operator()(Int) const {
        throw(std::out_of_range());
    }
};

template<class FusionSequence>
void print_nth(const FusionSequence& s, std::size_t n) {
    typedef typename fusion::result_of::size<FusionSequence>::type size;
    typedef mpl::range_c<std::size_t, 0, size::value> range;
    switch_<range>(n, print_nth_function(s), throw_out_of_range());
}

prints the nth element of a fusion sequence.