Toon Knapen wrote:

I have a list of heterogeneous objects, e.g.

struct Foo { Foo(int v) : value_( v ) {} int value() const { return value_ ; } int value_; };

struct Bar { Bar(int v) : value_( v ) {} int value() const { return value_ ; } int value_; };

int main() { Foo foo(3); Bar bar(7); typedef boost::tuple< Foo, Bar > MyTuple ; MyTuple my_tuple( foo, bar ); return 0; }

And now I want to have a function that can accumulate the value sof all the objects in the tuple, e.g. (pseudocode since my problem is exacly that I can't figure out how to code this)

template < typename TypleType > int accumulate_values(const TupleType& t) { int acc = 0; for(int i = 0 ; i < t.size() ; ++i ) acc += (get< i >( t )).value(); }

Can I do this using mpl (mpl looks great but I can't figure out how to do this).

It's not exactly a part of MPL (the library doesn't target "run-time" stuff to that extent yet; I hope it will some day), but "tuple extension" components I wrote some time ago (http://groups.yahoo.com/group/boost/files/tuple_ext/tuple_ext-08-nov-01.zip ) allow one to do exactly these kinds of things: // untested! :) template< typename Seq, typename T, typename BinaryOp > T accumulate(Seq const& s, T const& Init, BinaryOp op) { T result = Init; for_each( begin(s) , end(s) , boost::bind<T>(op, boost::ref(result), _1) ); return result; } With the above, you can do something like this: struct get_value { template< typename T > int operator()(T const& x) const { return x.value(); } }; int sum = accumulate( tuple , 0 , boost::bind(std::plus<int>() , _1 , boost::bind<int>(get_value(),_2) ) ); Hope it's not too complicated :). LL might allow to do better on this one. Aleksey