Should be fixed.

For c++.
The tests breaks if you remove "using namespace std;"
It should not depend on the user code having this using in it.

Either have the using namespace in the actual test. Or better, just type std:: where it is needed.

Not O(log N)
you are still sorting wich is O(N log N)

And O(2N) is O(N), in the usual notation.

tester("pneumonoultramicroscopicsilicovolcanoconiosis", "sisoinoconaclovociliscipocsorcimartluonomuenp");

Personal not a fan of using std. It shows more intent, and you avoid bugs with name collisions

Noice xD

Oh neat I assumed std::multiplies<void> only had one template parameter like the regular version does. Cool

It should work with different types

template< class T = void >
struct multiplies;
(since C++14)

and the void version is the type deducing one
std::multiplies<void> is a specialization of std::multiplies with parameter and return type deduced.