transform_iterator compile problem

Question

HI,

I don\'t like posting compile problems, but I really can\'t figure this one out. Using this code:

#include 

Answer 1

Since you also asked for an explanation

The transform_iterator needs to know the return type of the function called in order to instantiate itself. This is determined via result_of (found in <boost/utility/result_of.hpp>

If you use a function object, you need to define a member result_type to specify the result type of the object. (since an object doesn't have a 'return type' as such)

If you would have used a regular function, result_of would be able to figure it out on his own, e.g.:

template <typename K, typename V>
const V & get_value(std::pair<K, V> const & p)  { return p.second; }

class test
{
  typedef map<int, float> TMap;
  TMap mymap;

public:
  typedef boost::function< const TMap::mapped_type & (const  TMap::value_type &)  > F;
  typedef boost::transform_iterator<F, TMap::iterator> transform_iterator;

  transform_iterator begin()
  {
    return boost::make_transform_iterator(mymap.begin(), &get_value< int, float >);
  }
};

Answer 2

You'll have to inherit get_value from unary_function<const V&, std::pair<K, V> const&> to tell transform_iterator what the signature of get_value is.