# Extensions for parallelism

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< cpp‎ | experimental

The C++ Extensions for Parallelism, ISO/IEC TS 19570:2015 defines the following new components for the C++ standard library:

### Execution policies

The parallelism TS describes three execution policies: sequential, parallel, and parallel+vector, and provides corresponding execution policy types and objects. Users may select an execution policy statically by invoking a parallel algorithm with the an execution policy object of the corresponding type, or dynamically by using the type-erasing `execution_policy` class.

Implementations may define additional execution policies as an extension. The semantics of parallel algorithms invoked with an execution policy object of implementation-defined type is implementation-defined.

Defined in header `<experimental/execution_policy>`
sequential_execution_policy parallel_execution_policy parallel_vector_execution_policy | execution policy types (class)
seq par par_vec | global execution policy objects (constant)
execution_policy | dynamic execution policy (class)
is_execution_policy | test whether a class represents an execution policy (class template)

### Exception lists

Defined in header `<experimental/exception_list>`
exception_list | exceptions raised during parallel executions (class)

### Parallelized versions of existing algorithms

The TS provides parallelized versions of the following 69 algorithms from <algorithm>, <numeric> and <memory>:

Standard library algorithms for which parallelized versions are provided

- std::adjacent_difference
- std::adjacent_find
- std::all_of
- std::any_of
- std::copy
- std::copy_if
- std::copy_n
- std::count
- std::count_if
- std::equal
- std::fill
- std::fill_n
- std::find
- std::find_end
- std::find_first_of
- std::find_if
- std::find_if_not
- std::generate
- std::generate_n
- std::includes
- std::inner_product
- std::inplace_merge
- std::is_heap
- std::is_heap_until
- std::is_partitioned
- std::is_sorted
- std::is_sorted_until
- std::lexicographical_compare
- std::max_element
- std::merge
- std::min_element
- std::minmax_element
- std::mismatch
- `std::move`
- std::none_of
- std::nth_element
- std::partial_sort
- std::partial_sort_copy
- std::partition
- std::partition_copy
- `std::remove`
- std::remove_copy
- std::remove_copy_if
- std::remove_if
- std::replace
- std::replace_copy
- std::replace_copy_if
- std::replace_if
- std::reverse
- std::reverse_copy
- std::rotate
- std::rotate_copy
- std::search
- std::search_n
- std::set_difference
- std::set_intersection
- std::set_symmetric_difference
- std::set_union
- std::sort
- std::stable_partition
- std::stable_sort
- std::swap_ranges
- std::transform
- std::uninitialized_copy
- std::uninitialized_copy_n
- std::uninitialized_fill
- std::uninitialized_fill_n
- std::unique
- std::unique_copy

### New algorithms

Defined in header `<experimental/algorithm>`
for_each | similar to std::for_each except returns void (function template)
for_each_n | applies a function object to the first n elements of a sequence (function template)
Defined in header `<experimental/numeric>`
reduce (parallelism TS) | similar to std::accumulate, except out of order (function template)
exclusive_scan | similar to std::partial_sum, excludes the ith input element from the ith sum (function template)
inclusive_scan | similar to std::partial_sum, includes the ith input element in the ith sum (function template)
transform_reduce (parallelism TS) | applies a functor, then reduces out of order (function template)
transform_exclusive_scan | applies a functor, then calculates exclusive scan (function template)
transform_inclusive_scan | applies a functor, then calculates inclusive scan (function template)

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