async.h

Header-only parallel for-each library using std::async in C++

Lightweight task-based parallelism using modern C++ (C++20/23). Supports both dynamic runtime dispatch and compile-time loop unrolling.

• Parallel for_each dynamic dispatch over iterators
• Compile-time for_each dispatch over sequences
• Per-thread CPU pinning (Linux only)
• Optional progress reporting

Dynamic async_for_each dispatch over containers

Basic usage

	#include "async.h"

	std::vector<float> data(1000, 42);

	async::async_for_each(data.begin(), data.end(), [](float& x) { ... });  

Supported signatures

The lambda passed to async_for_each can have the following signatures:

Signature	Description
f(T& value)	The operand
f(T& value, size_t index)	Exposes the index of the operand
f(T& value, size_t index, size_t thread_id)	Exposes the thread id managing the operand

Exposing the thread id

	async::async_for_each(data.begin(), data.end(),
	[](int& x, size_t idx, size_t thread_id) 
	{
		std::cout << "Thread " << thread_id << " processing index " << idx << "\n";
	});

Compile-Time async_for_each dispatch over sequences

This version unrolls and partitions the index space at compile time.

	async::async_for_each<size_t, 0, 100, 1>(
	[](size_t i) 
	{
		std::cout << "index: " << i << "\n";
	});

Supported signatures

The lambda passed to async_for_each can have the following signatures:

Signature	Description
f(T index)	Basic index access
f(T index, size_t thread_id)	Access with thread ID

Example with thread ID:

	async::async_for_each<size_t, 0, 100, 2>(
	[](size_t i, size_t thread_id) 
	{
		std::cout << "Thread " << thread_id << " processing index " << i << "\n";
	});

Etc

Thread Pinning (Linux only)

On Linux, threads are pinned to CPU cores to improve cache locality.

Exception Safety

Exceptions in any thread are caught and rethrown in the calling thread. Execution aborts early on first exception.

Progress tracking

You can track how many threads have completed their chunk:

	std::atomic<size_t> progress = 0;

	async::async_for_each(data.begin(), data.end(), [](int& x) { x *= 2; }, async::threads,
	[&](size_t completed) 
	{
		std::cout << "Completed threads: " << completed << "\n";
	});