C++17 STL Cookbook

How do those execution policies work?

The execution policy tells which strategy we allow for the automatic parallelization of our standard algorithm calls.

The following three policy types exist in the std::execution namespace:

Policy	Meaning
sequenced_policy	The algorithm has to be executed in a sequential form similar to the original algorithm without an execution policy. The globally available instance has the name std::execution::seq.
parallel_policy	The algorithm may be executed with multiple threads that share the work in a parallel fashion. The globally available instance has the name std::execution::par.
parallel_unsequenced_policy	The algorithm may be executed with multiple threads sharing the work. In addition to ...