C++併發（利用線程分割任務）

example: 利用線程來分割任務

#include <iostream>
#include <thread>
#include<vector>
#include <algorithm>
#include <numeric>

template<typename Iterator, typename T>
struct accumulate_block
{
	void operator()(Iterator first, Iterator last, T& result)
	{
		result = std::accumulate(first, last, result);
	}
};

template<typename Iterator, typename T>
T parallel_accumulate(Iterator first, Iterator last, T init)
{
	unsigned long const length = std::distance(first, last);
	if (!length)
		return init;
	unsigned long const min_per_thread = 25;
	unsigned long const max_threads =
		(length + min_per_thread - 1) / min_per_thread;
	unsigned long const hardware_threads =
		std::thread::hardware_concurrency();
	unsigned long const num_threads =
		std::min(hardware_threads != 0 ? hardware_threads : 2, max_threads);
	unsigned long const block_size = length / num_threads;
	std::vector<T> results(num_threads);
	std::vector<std::thread> threads(num_threads - 1);
	Iterator block_start = first;
	for (unsigned long i = 0; i < (num_threads - 1); ++i)
	{
		Iterator block_end = block_start;
		std::advance(block_end, block_size);
		threads[i] = std::thread(
			accumulate_block<Iterator, T>(),
			block_start, block_end, std::ref(results[i])
		);
		block_start = block_end;
	}
	accumulate_block<Iterator, T>()(
		block_start, last, results[num_threads - 1]);
	std::for_each(threads.begin(), threads.end(),
		std::mem_fn(&std::thread::join));
	return std::accumulate(results.begin(), results.end(), init);
}

int main()
{
	std::vector<int> a(56, 2);
	int re;
	re = parallel_accumulate<std::vector<int>::iterator, int>(a.begin(), a.end(), 0);
	std::cout << re << std::endl;
	system("pause");
	return 0;
}