it is pass-by-value since the & was removed. So the input parameters are in fact copies. The code is fuly eqivalent to the function taking const& and then making explicit copies.

It is called a fold expression

I removed also the & to take a copy of the inputs. So from the callers perspective the signature is effectively the same.

Well, your solution has the same time-compelxity (unless s2 is much smaller than s1) but much worse chache-locality due to the use of std::multiset which is a node based structure.

The kata description says: "The input string can be assumed to contain only alphabets (both uppercase and lowercase) and numeric digits."

I've got the advice to never move in a return statement from Scott Meyer's 'Effective Modern C++'

It is a local variable since you are copy-constructing it.

It's a premature pessimization to move in a return statement. It prevents named return value optimization (NRVO). If NRVO were note possible the value would be moved either way since it is a so-called expiring lvalue.

So if you write return str the returned value will be either moved to its destination if NRVO is not supported or else the move will be elided away.

rust translation, please review and approve.

You are not allowed to use functions to solve this kata. The purpose is to train template metaprogramming.