std::execution::seq, std::execution::par, std::execution::par_unseq, std::execution::unseq

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Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
execution::seqexecution::parexecution::par_unseqexecution::unseq
(C++17)(C++17)(C++17)(C++20)  
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
C library
 
Defined in header <execution>
inline constexpr std::execution::sequenced_policy seq { /* unspecified */ };
(since C++17)
inline constexpr std::execution::parallel_policy par { /* unspecified */ };
(since C++17)
inline constexpr std::execution::parallel_unsequenced_policy par_unseq { /* unspecified */ };
(since C++17)
inline constexpr std::execution::unsequenced_policy unseq { /* unspecified */ };
(since C++20)

std::execution::seq, std::execution::par, std::execution::par_unseq, and std::execution::unseq are instances of the execution policy types std::execution::sequenced_policy, std::execution::parallel_policy, std::execution::parallel_unsequenced_policy, and std::execution::unsequenced_policy respectively. They are used to specify the execution policy of parallel algorithms - i.e., the kinds of parallelism allowed.

Additional execution policies may be provided by a standard library implementation (possible future additions may include std::parallel::cuda and std::parallel::opencl)

See also

execution policy types
(class)