Parallel STL Overview

Introduction to Parallel STL

Parallel STL is an implementation of the C++ standard library algorithms with support for execution policies, as specified in ISO/IEC 14882:2017 standard, commonly called C++17. The implementation also supports the unsequenced execution policy and the algorithms shift_left and shift_right, which are specified in the final draft for the C++ 20 standard (N4860). For more details see the C++ Reference Standard Execution Policies.

Parallel STL offers efficient support for both parallel and vectorized execution of algorithms for Intel® processors. For sequential execution, it relies on an available implementation of the C++ standard library.

The implementation supports the Data Parallel C++ (DPC++) execution policies used to run the massive parallel computational model for heterogeneous systems. The policies are specified in the oneAPI DPC++ Library (oneDPL) section of the oneAPI Specification.

For any of the implemented algorithms, pass one of the execution policies values as the first argument in a call to the algorithm to specify the desired execution policy. The policies have the following meaning:

Execution policy value



Sequential execution.


Unsequenced SIMD execution. This policy requires that all functions provided are SIMD-safe.


Parallel execution by multiple threads.


Combined effect of unseq and par.


Massive parallel execution on devices using DPC++.


Massive parallel execution on FPGA devices.

The implementation is based on Parallel STL from the LLVM Project.


C++11 is the minimal version of the C++ standard, which oneDPL requires. That means, any use of oneDPL requires at least a C++11 compiler. Some uses of the library may require a higher version of C++. To use Parallel STL with the C++ standard policies, you must have the following software installed:

  • C++ compiler with support for OpenMP* 4.0 (or higher) SIMD constructs

  • oneAPI Threading Building Blocks (oneTBB) or Intel® Threading Building Blocks (Intel® TBB) 2019 and later

To use Parallel STL with the DPC++ execution policies, you must have the following software installed:

  • C++ compiler with support for SYCL* 2020