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 <map>
#include <boost/iterator/transform_iterator.hpp>

using namespace std;

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

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

public:
    typedef get_value<TMap::key_type, TMap::value_type> F;
    typedef boost::transform_iterator<F, TMap::iterator> transform_iterator;

    transform_iterator begin()
    {
     return make_transform_iterator(mymap.begin(), F());
    }
};

Getting this compile error:

transform_iterator.hpp(43) : error C2039: 'result_type' : is not a member of 'get_value<K,V>'
        with
        [
            K=int,
            V=std::pair<const int,float>
        ]

Can anyone explain why this isn't working? I'm using Visual Studio 7.0 with boost 1.36.0

Thanks.

Answer 1

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.

Answer 2

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 >);
  }
};