Obtaining list of keys and values from unordered_map

Question

What is the most efficient way of obtaining lists (as a vector) of the keys and values from an unordered_map?

For concreteness, suppose the

Answer 1

Using C++-14 you could also do the following (edited to contain full source):

#include 
#include 
#include 
#include 
#include 

using namespace std;

typedef string Key;
typedef int Value;

auto key_selector = [](auto pair){return pair.first;};
auto value_selector = [](auto pair){return pair.second;};

int main(int argc, char** argv) {
  // Create a test map
  unordered_map map;
  map["Eight"] = 8;
  map["Ten"] = 10;
  map["Eleven"] = 11;

  // Vectors to hold keys and values
  vector keys(map.size());
  vector values(map.size());

  // This is the crucial bit: Transform map to list of keys (or values)
  transform(map.begin(), map.end(), keys.begin(), key_selector);
  transform(map.begin(), map.end(), values.begin(), value_selector);

  // Make sure this worked: Print out vectors
  for (Key key : keys) cout << "Key: " << key << endl;
  for (Value value : values) cout << "Value: " << value << endl;

  return 0;
}

I compiled this with the following command:

g++ keyval.cpp -std=c++14 -o keyval

Testing it printed the keys and values as expected.