std::experimental::parallel::seq, std::experimental::parallel::par, std::experimental::parallel::par_vec
From cppreference.com
                    
                                        
                    < cpp | experimental
                    
                                                            
                    | Defined in header  <experimental/execution_policy> | ||
| constexpr sequential_execution_policy seq{}; | ||
| constexpr parallel_execution_policy par{}; | ||
| constexpr parallel_vector_execution_policy par_vec{}; | ||
seq, par and par_vec are instances of the execution policy types sequential_execution_policy, parallel_execution_policy and parallel_vector_execution_policy respectively. They are used to specify the execution policy of parallel algorithms - i.e., the kinds of parallelism allowed.


