# <execution>

**[<execution>](https://en.cppreference.com/w/cpp/header/execution)** provides execution policies for parallel algorithms introduced in C++17: `std::execution::seq` (sequential), `std::execution::par` (parallel), `std::execution::par_unseq` (parallel, unsequenced), `std::execution::unseq` (unsequenced, C++20).

These policies are passed to algorithms like `std::sort`, `std::for_each`, and `std::transform` to request parallelization. The standard library or compiler runtime chooses how many threads to use.

## Example

This example sorts a vector using the `std::execution::par` policy to enable parallel sorting across multiple threads.

```cpp
// compile: g++ -std=c++17 -pthread -O3 -o executionexample executionexample.cpp
// run: ./executionexample
// description: parallel sort using execution policy

#include <execution>
#include <algorithm>
#include <vector>
#include <iostream>
#include <random>

int main() {
    std::vector<int> v(1000);
    std::iota(v.begin(), v.end(), 0);
    std::shuffle(v.begin(), v.end(), std::mt19937{});
    
    std::sort(std::execution::par, v.begin(), v.end());
    
    std::cout << "first 5: ";
    for (int i = 0; i < 5; ++i) std::cout << v[i] << " ";
    std::cout << "\n";
    
    return 0;
}
```