線程 thread
(支持多線程編程,需要特定的庫的支持)
-
屬於某個平臺的多線程庫:
POSIX線程庫(Linux),Windows線程庫(Windows),三方數據庫(Boost線程庫) -
C++ 11標準支持多線程:
- < thread > : 包含std::thread類以及std::this_thread命名空間。管理線程的函數和類在該頭文件中有聲明;
- < atomic> :包含std::atomic和std::atomic_flag類,以及一套C風格的原子類型和與C兼容的原子操作的函數;
- < mutex> :包含了與互斥量相關的類以及其他類型的函數;
- < future >: 包含兩個Provider類(std::promise和std::package_task)和兩個Future類(std::future和std::shared_future)以及相關的類型和函數;
- < condition_variable > : 包含與條件變量相關的類,包括std::condition_variable和std::condition_variable_any
(本文介紹基於C++11一個簡單線程池的實現)
C++11線程池
#ifndef THREAD_POOL_H
#define THREAD_POOL_H
#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>
class ThreadPool {
public:
ThreadPool(size_t);
template<class F, class... Args>
auto enqueue(F&& f, Args&&... args)
-> std::future<typename std::result_of<F(Args...)>::type>;
~ThreadPool();
private:
// 需要跟蹤線程,以便我們可以加入它們
std::vector< std::thread > workers;
// 任務隊列
std::queue< std::function<void()> > tasks;
// 同步
std::mutex queue_mutex;
std::condition_variable condition;
bool stop;
};
// 構造函數只啓動一些worker
inline ThreadPool::ThreadPool(size_t threads)
: stop(false)
{
for(size_t i = 0;i<threads;++i)
workers.emplace_back(
[this]
{
for(;;)
{
std::function<void()> task;
{
std::unique_lock<std::mutex> lock(this->queue_mutex);
this->condition.wait(lock,
[this]{ return this->stop || !this->tasks.empty(); });
if(this->stop && this->tasks.empty())
return;
task = std::move(this->tasks.front());
this->tasks.pop();
}
task();
}
}
);
}
//向池中添加新的工作項
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args)
-> std::future<typename std::result_of<F(Args...)>::type>
{
using return_type = typename std::result_of<F(Args...)>::type;
auto task = std::make_shared< std::packaged_task<return_type()> >(
std::bind(std::forward<F>(f), std::forward<Args>(args)...)
);
std::future<return_type> res = task->get_future();
{
std::unique_lock<std::mutex> lock(queue_mutex);
// 停止池後不允許排隊
if(stop)
throw std::runtime_error("enqueue on stopped ThreadPool");
tasks.emplace([task](){ (*task)(); });
}
condition.notify_one();
return res;
}
// 析構函數連接所有線程
inline ThreadPool::~ThreadPool()
{
{
std::unique_lock<std::mutex> lock(queue_mutex);
stop = true;
}
condition.notify_all();
for(std::thread &worker: workers)
worker.join();
}
#endif
關於上面線程池的基本用法:
// 創建4個工作線程的線程池
ThreadPool pool(4);
// 放入隊列存儲
auto result = pool.enqueue([](int answer) { return answer; }, 42);
// 得到結果
std::cout << result.get() << std::endl;
簡單用例:
#include <iostream>
#include <vector>
#include <chrono>
#include "ThreadPool.h"
int main()
{
ThreadPool pool(4);
std::vector< std::future<int> > results;
for(int i = 0; i < 8; ++i) {
results.emplace_back(
pool.enqueue([i] {
std::cout << "hello " << i << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(1));
std::cout << "world " << i << std::endl;
return i*i;
})
);
}
for(auto && result: results)
std::cout << result.get() << ' ';
std::cout << std::endl;
return 0;
}