Nice insight on how to correctly write a boost::transform_iterator for std::map<K,V>. The thing to keep in mind is that std::map<K,V>::value_type is in fact std::pair<K const, V> instead of std::pair<K, V>. The interesting snippet from the best answer reads:

The problem is that the functor object takes a reference to pair<K,V>, but is being passed a reference to the map's value type, pair<const K,V>. This requires a conversion, which requires copying both the key and the value to a new pair.

This fact could be crucial when using non-copyable objects, e.g., std::unique_ptr as keys. The correct function object is as follows:

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

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