Simple thread pool using only standard library components. Also includes a class for a priority thread pool.

Requires concepts and C++20. Currently only GCC 10.0+ is sufficient. Use -std=c++20 -fconcepts to compile.

The priority thread pool is only supported on POSIX/-like systems. But it's still easy to use the normal pool on non-POSIX; just don't compile priority_thread_pool.cpp or include the header.

For just C++11, use 8bdfb9b. 5ea01d0 was the latest to support <= C++14. For C++17, use e3be25 and compile with -std=c++17 -fconcepts.

The priority pool has the same API as described below, accept it has an int parameter first for the priority of the task. E.g. pool.async(5, func, arg1, arg2) for priority 5.

An example that computes the primality of 2 to 10,000 using 8 threads:

#include "thread_pool.hpp"

#include <iostream>
#include <list>
#include <utility>

using namespace std;

// Return the integer argument and a boolean representing its primality.
// We need to return the integer because the loop that retrieves results doesn't know which integer corresponds to which future.
pair<int, bool> is_prime(int n){
  for(int i = 2;i < n;i++)
    if(n % i == 0)
      return make_pair(i, false);
  return make_pair(n, true);
}

int main(){
  thread_pool pool(8);                   // Contruct a thread pool with 8 threads.
  list<future<pair<int, bool>>> results;
  for(int i = 2;i < 10000;i++){
  	// Add a task to the queue.
    results.push_back(pool.async(is_prime, i));
  }
  
  for(auto i = results.begin();i != results.end();i++){
    pair<int, bool> result = i->get();  // Get the pair<int, bool> from the future<...>
    cout << result.first << ": " << (result.second ? "is prime" : "is composite") << endl;
  }
  return 0;
}

thread_pool::async is a templated method that accepts any std::function and arguments to pass to the function. It returns a std::future<Ret> where Ret is the return type of the aforementioned std::function.

To submit a task: future<Ret> fut(pool.async(func, args...));.

To wait on fut to complete: Ret result = fut.get();