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std::execution::sequenced_policy, std::execution::parallel_policy, std::execution::parallel_unsequenced_policy

Defined in header `<execution>`
class sequenced_policy { /* unspecified */ };	(1)	(since C++17)
class parallel_policy { /* unspecified */ };	(2)	(since C++17)
class parallel_unsequenced_policy { /* unspecified */ };	(3)	(since C++17)

1) The 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. The invocations of element access functions in parallel algorithms invoked with this policy (usually specified as std::execution::seq) are indeterminately sequenced in the calling thread.

2) The execution policy type used as a unique type to disambiguate parallel algorithm overloading and indicate that a parallel algorithm's execution may be parallelized. The invocations of element access functions in parallel algorithms invoked with this policy (usually specified as std::execution::par) are permitted to execute in either the invoking thread or in a thread implicitly created by the library to support parallel algorithm execution. Any such invocations executing in the same thread are indeterminately sequenced with respect to each other.

3) The 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. The invocations of element access functions in parallel algorithms invoked with this policy are permitted to execute in an unordered fashion in unspecified threads, and unsequenced with respect to one another within each thread.

Notes

When using parallel execution policy, it is the programmer's responsility to avoid deadlocks:

int a[] = {0,1};
std::vector<int> v;
std::for_each(std::execution::par, std::begin(a), std::end(a), [&](int i) {
  v.push_back(i*2+1); // Error: data race
});

std::atomic<int> x{0};
int a[] = {1,2};
std::for_each(std::execution::par, std::begin(a), std::end(a), [&](int) {
  x.fetch_add(1, std::memory_order_relaxed);
  while (x.load(std::memory_order_relaxed) == 1) { } // Error: assumes execution order
});

int x = 0;
std::mutex m;
int a[] = {1,2};
std::for_each(std::execution::par, std::begin(a), std::end(a), [&](int) {
  std::lock_guard<std::mutex> guard(m);
  ++x; // correct
});

Unsequenced execution policy is the only case where function calls are unsequenced with respect to each other, meaning they can be interleaved. In all other situations in C++, they are indeterminately-sequenced (cannot interleave). Because of that, users are not allowed to allocate or deallocate memory, acquire mutexes or perform any other vectorization-unsafe operations when using this policy (vectorization-unsafe functions are the ones that synchronize-with another function, e.g. std::mutex::unlock synchronizes-with the next std::mutex::lock).

int x = 0;
std::mutex m;
int a[] = {1,2};
std::for_each(std::execution::par_unseq, std::begin(a), std::end(a), [&](int) {
  std::lock_guard<mutex> guard(m); // Error: lock_guard constructor calls m.lock()
  ++x;
});

std::execution::sequenced_policy, std::execution::parallel_policy, std::execution::parallel_unsequenced_policy

Notes

See also