Question

I’m having problems using boost::transform_iterators where “ordinary” iterators are expected. For instance, I want to add all keys from a map to a set. I wrote the following short snippet:

template <typename K, typename V>
struct map_keys {
    typedef const K& result_type;
    const K& operator()(const std::pair<K,V>& kvp) const {
        return kvp.first;
    }
};

int main() {
    std::map<int, double> my_map;
    std::set<int> my_set;
    my_map[1]=1.2;
    my_map[2]=2.4;
    my_map[4]=4.1;
    my_map[6]=12.2;
    my_map[123]=3;
    typedef map_keys<int, double> mk;
    auto b = boost::make_transform_iterator(my_map.begin(), mk()),
        e = boost::make_transform_iterator(my_map.end(), mk());
    my_set.insert(b,e);
    return 0;
}

After the insert, my_set contains a single value, -858993460 or 0xcccccccc. Why? If I print *b in a loop, all the values are printed as expected.

Answer 1

Ok, solved it myself suddenly. The problem is the typedef result_type (which is not mentioned in the documentation, but unless I have it, the code does not compile (result_type is not a member of map_keys<K,V>)). If I instead typedef it as

typedef K result_type;

It works. I suppose as it was written earlier, it returned the address of something?