(C++11) Coroutine library.
boost.coroutine2 provides templates for generalized subroutines which allow multiple entry points for suspending and resuming execution at certain locations. It preserves the local state of execution and allows re-entering subroutines more than once (useful if state must be kept across function calls).
Coroutines can be viewed as a language-level construct providing a special kind of control flow.
In contrast to threads, which are pre-emptive, coroutines switches are cooperative (programmer controls when a switch will happen). The kernel is not involved in the coroutine switches.
boost.coroutine2 requires C++11! Note that boost.coroutine2 is the successor of the deprectated boost.coroutine.