How to retrieve all keys (or values) from a std::map and put them into a vector?

This is one of the possible ways I come out:

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

map m;
vector keys;

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

// Dump all keys
copy(keys.begin(), keys.end(), ostream_iterator(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.)

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:

std::map m;
std::vector key, value;
for(std::map::iterator it = m.begin(); it != m.end(); ++it) {
  key.push_back(it->first);
  value.push_back(it->second);
  std::cout first second << std::endl;
}

Or even simpler, if you are using the Boost library:

map m;
pair me; // what a map is made of
vector 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.

Stack Overflow Q#110157, score: 241