Is it possible to iterate an mpl::vector at run time without instantiating the types in the vector?

Question

Generally, I would use boost::mpl::for_each<>() to traverse a boost::mpl::vector, but this requires a functor with a template function declar

Answer 1

Interesting question! As far as I can tell, Boost.MPL does not seem to provide such an algorithm. However, writing your own should not be too difficult using iterators.

Here is a possible solution:

#include 
#include 
#include 

using namespace boost::mpl;


namespace detail {

template < typename Begin, typename End, typename F >
struct static_for_each
{
    static void call( )
    {
        typedef typename Begin::type currentType;

        F::template call< currentType >();
        static_for_each< typename next< Begin >::type, End, F >::call();
    }
};


template < typename End, typename F >
struct static_for_each< End, End, F >
{
    static void call( )
    {
    }
};

} // namespace detail


template < typename Sequence, typename F >
void static_for_each( )
{
    typedef typename begin< Sequence >::type begin;
    typedef typename end< Sequence >::type   end;

    detail::static_for_each< begin, end, F >::call();
}

[The naming may not be very well chosen, but well...]

Here is how you would use this algorithm:

struct Foo
{
    static void staticMemberFunction( )
    {
        std::cout << "Foo";
    }
};


struct Bar
{
    static void staticMemberFunction( )
    {
        std::cout << "Bar";
    }
};


struct CallStaticMemberFunction
{
    template < typename T >
    static void call()
    {
        T::staticMemberFunction();
    }
};


int main()
{
    typedef vector< Foo, Bar > sequence;

    static_for_each< sequence, CallStaticMemberFunction >(); // prints "FooBar"
}