How to Efficiently Extract Keys or Values from a std::map into a Vector?

Retrieve Elements from std::map into Vector: Beyond Functors

To extract keys or values from a std::map, one might instinctively employ functors as demonstrated in the provided code snippet:

struct RetrieveKey
{
    template <typename T>
    typename T::first_type operator()(T keyValuePair) const
    {
        return keyValuePair.first;
    }
};

map<int, int> m;
vector<int> keys;
transform(m.begin(), m.end(), back_inserter(keys), RetrieveKey());

While this approach technically functions, it suffers from code obscuration and execution distance from the target.

Alternative Method: Iterator Loop

A more straightforward solution involves a simple loop over the map's iterators:

for (map<int, int>::iterator it = m.begin(); it != m.end(); ++it) {
  keys.push_back(it->first);
}

This method offers clear readability and easy modification to retrieve values instead of keys.

Boost Library Approach

If using the Boost library, one can employ BOOST_FOREACH for even greater simplicity:

pair<int, int> me; // map element type
vector<int> v;
BOOST_FOREACH(me, m) {
  v.push_back(me.first);
}

This approach provides explicitness and conciseness.

In summary, while the functor method remains a valid option, the iterator loop and Boost library approaches offer simpler and more intuitive ways to retrieve elements from a std::map into a vector.

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