You don't know when something comes to hot path.

    if (std::set(itr.begin(), itr.end()).count(element)) { _____ } 

is it tempting for someone than something like

std::find(itr.begin(), itr.end(), element) != itr.end())

?? I don't know. That said, C++ STL quite undiscoverable.

>You don't know when something comes to hot path.

That's the point of the first advice though...