RE: [Boost-Users] iterating over a tuple

26 Apr 2002

      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

RE: [Boost-Users] iterating over a tuple

Aleksey Gurtovoy