Prevent unnecessary copies of C++ functor objects

Question

I have a class which accumulates information about a set of objects, and can act as either a functor or an output iterator. This allows me to do things like:

Answer 1

Just a quick note, for_each, accumulate, transform (2nd form), provide no order guarantee when traversing the provided range.

This makes sense for implementers to provide mulit-threaded/concurrent versions of these functions.

Hence it is reasonable that the algorithm be able to provide an equivalent instance (a new copy) of the functor passed in.

Be wary when making stateful functors.