Owen Owen - 3 months ago 28
C++ Question

How to retrieve all keys (or values) from a std::map?

This is one of the possible ways I come out:

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

map<int, int> m;
vector<int> keys;

// Retrieve all keys
transform(m.begin(), m.end(), back_inserter(keys), RetrieveKey());

// Dump all keys
copy(keys.begin(), keys.end(), ostream_iterator<int>(cout, "\n"));


Of course, we can also retrieve all values from the map by defining another functor RetrieveValues.

Is there any other way to achieve this easily? (I'm always wondering why std::map does not include a member function for us to do so.)

Answer

While your solution should work, it can be difficult to read depending on the skill level of your fellow programmers. Additionally, it moves functionality away from the call site. Which can make maintenance a little more difficult.

I'm not sure if your goal is to get the keys into a vector or print them to cout so I'm doing both. You may try something like this:

map<int, int> m;
vector<int> v;
for(map<int,int>::iterator it = m.begin(); it != m.end(); ++it) {
  v.push_back(it->first);
  cout << it->first << "\n";
}

Or even simpler, if you are using Boost:

map<int,int> m;
pair<int,int> me; // what a map<int, int> is made of
vector<int> v;
BOOST_FOREACH(me, m) {
  v.push_back(me.first);
  cout << me.first << "\n";
}

Personally, I like the BOOST_FOREACH version because there is less typing and it is very explicit about what it is doing.