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.
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 >);
}
};
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.