Question

I know this questions has come up in various guises before, but this is slightly different.

I have a class which contains a std::map. Although I wish to use the map for other purposes inside the class, externally I want to expose an iterator adapter to just the values inside the map (ie the second item in the std::pair).

For example in python I might do something like this:

def __iter__(self):     return self._dict.itervalues() 

How do I go about doing this in c++, hiding the implementation inside the class?

Thanks,

Dan

Answer 1

Have a look at Boost’s transform_iterator which provides exactly this kind of functionality:

template <typename K, typename V> struct get_value {     const V& operator ()(std::pair<K, V> const& p) { return p.second; } };  class your_class {     typedef map<int, float> TMap;     TMap mymap;  public:     typedef get_value<TMap::key_type, TMap::data_type> F;     typedef         boost::transform_iterator<F, TMap::iterator>         value_iterator;      value_iterator begin() { return make_transform_iterator(mymap.begin(), F()); }      value_iterator end() { return make_transform_iterator(mymap.end(), F()); }      // TODO Same for const versions.     // Rest of the interface … }; 

Now you can iterate over the values, e.g. like this:

your_class c; // Fill c with some values … copy(c.begin(), c.end(), ostream_iterator<float>(cout, ' '));