std::execution::par

std::execution::par

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Prerequisites

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1. What is std::execution::par? (C++17 Parallel Execution)

C++17 introduced parallel algorithms through the <execution> header. std::execution::par is a policy that enables multi-threaded execution for standard algorithms.

It allows developers to write code in a familiar STL style while letting the standard library handle parallelization internally.

#include <algorithm>
#include <execution>
#include <vector>

std::vector<int> v = {1, 2, 3, 4, 5};

std::for_each(std::execution::par, v.begin(), v.end(), [](int& x)
{
    x *= 2;
});

{2, 4, 6, 8, 10}

2. How to work

std::execution::par

This single argument changes the execution model:

• Sequential → Parallel
• Single thread → Multiple threads
• Manual thread management → Automatic

Conceptual Model

Internally, the algorithm may split work like this:

Thread 1 → v[0 ~ 249]
Thread 2 → v[250 ~ 499]
Thread 3 → v[500 ~ 749]
Thread 4 → v[750 ~ 999]

Each thread processes a different chunk independently.

Note: The actual implementation is library-dependent.

Supported Algorithms

Common algorithms that support par:

• std::for_each
• std::transform
• std::sort
• std::reduce
• std::transform_reduce
• std::inclusive_scan
• std::exclusive_scan

Example: transform

#include <algorithm>
#include <execution>
#include <vector>

std::vector<int> input = {1, 2, 3, 4, 5};
std::vector<int> output(input.size());

std::transform(std::execution::par,
               input.begin(), input.end(),
               output.begin(),
               [](int x)
               {
                   return x * x;
               });

Critical Rule: No Data Dependency

Parallel execution requires independent work per element.

3. Features

Safe

std::for_each(std::execution::par, v.begin(), v.end(), [](int& x)
{
    x *= 2;
});

• No shared state
• No dependency between elements

Unsafe (Race Condition)

int sum = 0;

std::for_each(std::execution::par, v.begin(), v.end(), [&](int x)
{
    sum += x;   // race condition
});

• Multiple threads modify sum
• Undefined behavior

Order is NOT Guaranteed

std::for_each(std::execution::par, v.begin(), v.end(), [](int x)
{
    std::cout << x << " ";
});

Output order may vary.