Keys / Values Functionality to Iterators in C++

Question

Hi,

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, " "));