Additions and Changes to Parallel Algorithms

Most algorithms in the standard library are available as parallel versions straight out the box. However, there are some noteworthy exceptions, including std::accumulate() and std::for_each(), as their original specifications required in-order execution.

std::accumulate() and std::reduce()

The std::accumulate() algorithm cannot be parallelized as it must be executed in the order of the elements, which is not possible to parallelize. Instead, a new algorithm called std::reduce() has been added, which works just like std::accumulate() with the exception that it is executed unordered.

With commutative operations, their results are the same, as the order of accumulation doesn’t matter. In other words, given a range of integers: