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

Question

I understand the reasons why one can't just do this (rebalancing and stuff):

iterator i = m.find(33);

if (i != m.end())
  i->first = 22;

But so far the only way (I know about) to change the key is to remove the node from the tree alltogether and then insert the value back with a different key:

iterator i = m.find(33);

if (i != m.end())
{
  value = i->second;
  m.erase(i);
  m[22] = value;
}

This seems rather inefficient to me for more reasons:

1. Traverses the tree three times (+ balance) instead of twice (+ balance)

2. One more unnecessary copy of the value

3. Unnecessary deallocation and then re-allocation of a node inside of the tree

I find the allocation and deallocation to be the worst from those three. Am I missing something or is there a more efficient way to do that?

I think, in theory, it should be possible, so I don't think changing for a different data structure is justified. Here is the pseudo algorithm I have in mind:

1. Find the node in the tree whose key I want to change.

2. Detach if from the tree (don't deallocate)

3. Rebalance

4. Change the key inside the detached node

5. Insert the node back into the tree

6. Rebalance

Answer 1

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" } }