What is the fastest way to change a key of an element inside std::map

In C++17, the new map::extract function lets you change the key.
Example:

std::map<int, std::string> m{ {10, "potato"}, {1, "banana"} };
auto nodeHandler = m.extract(10);
nodeHandler.key() = 2;
m.insert(std::move(nodeHandler)); // { { 1, "banana" }, { 2, "potato" } }

I proposed your algorithm for the associative containers about 18 months ago here:

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#839

Look for the comment marked: [ 2009-09-19 Howard adds: ].

At the time, we were too close to FDIS to consider this change. However I think it very useful (and you apparently agree), and I would like to get it in to TR2. Perhaps you could help by finding and notifying your C++ National Body representative that this is a feature you would like to see.

Update

It is not certain, but I think there is a good chance we will see this feature in C++17! :-)

You can omit the copying of value;

const int oldKey = 33;
const int newKey = 22;
const iterator it = m.find(oldKey);
if (it != m.end()) {
  // Swap value from oldKey to newKey, note that a default constructed value 
  // is created by operator[] if 'm' does not contain newKey.
  std::swap(m[newKey], it->second);
  // Erase old key-value from map
  m.erase(it);
}