[execpol]

# 26 Algorithms library [[algorithms]](./#algorithms)

## 26.3 Parallel algorithms [[algorithms.parallel]](algorithms.parallel#execpol)

### 26.3.6 Execution policies [execpol]

#### [26.3.6.1](#general) General [[execpol.general]](execpol.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L636)

Subclause [execpol] describes classes that are [*execution policy*](#def:execution_policy "26.3.6.1 General [execpol.general]") types[.](#general-1.sentence-1)

An
object of an execution policy type indicates the kinds of parallelism allowed
in the execution of an algorithm and expresses the consequent requirements on
the element access functions[.](#general-1.sentence-2)

Execution policy types are declared in header [<execution>](execution.syn#header:%3cexecution%3e "33.4 Header <execution> synopsis [execution.syn]")[.](#general-1.sentence-3)

[*Example [1](#general-example-1)*: using namespace std;
vector<int> v = /* ... */;

// standard sequential sort sort(v.begin(), v.end());

// explicitly sequential sort sort(execution::seq, v.begin(), v.end());

// permitting parallel execution sort(execution::par, v.begin(), v.end());

// permitting vectorization as well sort(execution::par_unseq, v.begin(), v.end()); — *end example*]

[*Note [1](#general-note-1)*:

Implementations can provide additional execution policies
to those described in this document as extensions
to address parallel architectures that require idiosyncratic
parameters for efficient execution[.](#general-1.sentence-4)

— *end note*]

#### [26.3.6.2](#type) Execution policy type trait [[execpol.type]](execpol.type)

[🔗](#lib:is_execution_policy)

`template<class T> struct is_execution_policy;
`

[1](#type-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L675)

is_execution_policy can be used to detect execution policies for the
purpose of excluding function signatures from otherwise ambiguous overload
resolution participation[.](#type-1.sentence-1)

[2](#type-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L680)

is_execution_policy<T> is a *Cpp17UnaryTypeTrait* with a
base characteristic of true_type if T is the type of a standard
or implementation-defined
execution policy, otherwise false_type[.](#type-2.sentence-1)

[*Note [1](#type-note-1)*:

This provision reserves the privilege of creating non-standard execution
policies to the library implementation[.](#type-2.sentence-2)

— *end note*]

[3](#type-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L691)

The behavior of a program that adds specializations foris_execution_policy is undefined[.](#type-3.sentence-1)

#### [26.3.6.3](#seq) Sequenced execution policy [[execpol.seq]](execpol.seq)

[🔗](#lib:execution::sequenced_policy)

`class execution::sequenced_policy { unspecified };
`

[1](#seq-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L704)

The class execution​::​sequenced_policy is an execution policy type used
as a unique type to disambiguate parallel algorithm overloading and require
that a parallel algorithm's execution may not be parallelized[.](#seq-1.sentence-1)

[2](#seq-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L709)

During the execution of a parallel algorithm with
the execution​::​sequenced_policy policy,
if the invocation of an element access function exits via an exception,terminate is invoked ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#seq-2.sentence-1)

#### [26.3.6.4](#par) Parallel execution policy [[execpol.par]](execpol.par)

[🔗](#lib:execution::parallel_policy)

`class execution::parallel_policy { unspecified };
`

[1](#par-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L724)

The class execution​::​parallel_policy is an execution policy type used as
a unique type to disambiguate parallel algorithm overloading and indicate that
a parallel algorithm's execution may be parallelized[.](#par-1.sentence-1)

[2](#par-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L729)

During the execution of a parallel algorithm with
the execution​::​parallel_policy policy,
if the invocation of an element access function exits via an exception,terminate is invoked ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#par-2.sentence-1)

#### [26.3.6.5](#parunseq) Parallel and unsequenced execution policy [[execpol.parunseq]](execpol.parunseq)

[🔗](#lib:execution::parallel_unsequenced_policy)

`class execution::parallel_unsequenced_policy { unspecified };
`

[1](#parunseq-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L744)

The class execution​::​parallel_unsequenced_policy is an execution policy type
used as a unique type to disambiguate parallel algorithm overloading and
indicate that a parallel algorithm's execution may be parallelized and
vectorized[.](#parunseq-1.sentence-1)

[2](#parunseq-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L750)

During the execution of a parallel algorithm with
the execution​::​parallel_unsequenced_policy policy,
if the invocation of an element access function exits via an exception,terminate is invoked ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#parunseq-2.sentence-1)

#### [26.3.6.6](#unseq) Unsequenced execution policy [[execpol.unseq]](execpol.unseq)

[🔗](#lib:execution::unsequenced_policy)

`class execution::unsequenced_policy { unspecified };
`

[1](#unseq-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L765)

The class unsequenced_policy is an execution policy type
used as a unique type to disambiguate parallel algorithm overloading and
indicate that a parallel algorithm's execution may be vectorized,
e.g., executed on a single thread using instructions
that operate on multiple data items[.](#unseq-1.sentence-1)

[2](#unseq-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L772)

During the execution of a parallel algorithm with
the execution​::​unsequenced_policy policy,
if the invocation of an element access function exits via an exception,terminate is invoked ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#unseq-2.sentence-1)

#### [26.3.6.7](#objects) Execution policy objects [[execpol.objects]](execpol.objects)

[🔗](#lib:seq)

`inline constexpr execution::sequenced_policy            execution::seq{ unspecified };
inline constexpr execution::parallel_policy             execution::par{ unspecified };
inline constexpr execution::parallel_unsequenced_policy execution::par_unseq{ unspecified };
inline constexpr execution::unsequenced_policy          execution::unseq{ unspecified };
`

[1](#objects-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L795)

The header [<execution>](execution.syn#header:%3cexecution%3e "33.4 Header <execution> synopsis [execution.syn]") declares global objects associated with each type of execution policy[.](#objects-1.sentence-1)