C++11 加入了线程库,从此告别了标准库不支持并发的历史。然而 c++ 对于多线程的支持还是比较低级,稍微高级一点的用法都需要自己去实现,譬如线程池、信号量等。线程池(thread pool)这个东西,在面试上多次被问到,一般的回答都是:“管理一个任务队列,一个线程队列,然后每次取一个任务分配给一个线程去做,循环往复。” 貌似没有问题吧。但是写起程序来的时候就出问题了。
废话不多说,先上实现,然后再啰嗦。(dont talk, show me ur code !)
代码实现
#pragma once #ifndef THREAD_POOL_H #define THREAD_POOL_H #include <vector> #include <queue> #include <thread> #include <atomic> #include <condition_variable> #include <future> #include <functional> #include <stdexcept> namespace std { #define MAX_THREAD_NUM 256 //线程池,可以提交变参函数或拉姆达表达式的匿名函数执行,可以获取执行返回值 //不支持类成员函数, 支持类静态成员函数或全局函数,Opteron()函数等 class threadpool { using Task = std::function<void()>; // 线程池 std::vector<std::thread> pool; // 任务队列 std::queue<Task> tasks; // 同步 std::mutex m_lock; // 条件阻塞 std::condition_variable cv_task; // 是否关闭提交 std::atomic<bool> stoped; //空闲线程数量 std::atomic<int> idlThrNum; public: inline threadpool(unsigned short size = 4) :stoped{ false } { idlThrNum = size < 1 ? 1 : size; for (size = 0; size < idlThrNum; ++size) { //初始化线程数量 pool.emplace_back( [this] { // 工作线程函数 while(!this->stoped) { std::function<void()> task; { // 获取一个待执行的 task std::unique_lock<std::mutex> lock{ this->m_lock };// unique_lock 相比 lock_guard 的好处是:可以随时 unlock() 和 lock() this->cv_task.wait(lock, [this] { return this->stoped.load() || !this->tasks.empty(); } ); // wait 直到有 task if (this->stoped && this->tasks.empty()) return; task = std::move(this->tasks.front()); // 取一个 task this->tasks.pop(); } idlThrNum--; task(); idlThrNum++; } } ); } } inline ~threadpool() { stoped.store(true); cv_task.notify_all(); // 唤醒所有线程执行 for (std::thread& thread : pool) { //thread.detach(); // 让线程“自生自灭” if(thread.joinable()) thread.join(); // 等待任务结束, 前提:线程一定会执行完 } } public: // 提交一个任务 // 调用.get()获取返回值会等待任务执行完,获取返回值 // 有两种方法可以实现调用类成员, // 一种是使用 bind: .commit(std::bind(&Dog::sayHello, &dog)); // 一种是用 mem_fn: .commit(std::mem_fn(&Dog::sayHello), &dog) template<class F, class... Args> auto commit(F&& f, Args&&... args) ->std::future<decltype(f(args...))> { if (stoped.load()) // stop == true ?? throw std::runtime_error("commit on ThreadPool is stopped."); using RetType = decltype(f(args...)); // typename std::result_of<F(Args...)>::type, 函数 f 的返回值类型 auto task = std::make_shared<std::packaged_task<RetType()> >( std::bind(std::forward<F>(f), std::forward<Args>(args)...) ); // wtf ! std::future<RetType> future = task->get_future(); { // 添加任务到队列 std::lock_guard<std::mutex> lock{ m_lock };//对当前块的语句加锁 lock_guard 是 mutex 的 stack 封装类,构造的时候 lock(),析构的时候 unlock() tasks.emplace( [task]() { // push(Task{...}) (*task)(); } ); } cv_task.notify_one(); // 唤醒一个线程执行 return future; } //空闲线程数量 int idlCount() { return idlThrNum; } }; } #endif
代码不多吧,上百行代码就完成了 线程池, 并且, 看看 commit, 哈, 不是固定参数的, 无参数数量限制! 这得益于可变参数模板.
怎么使用?
看下面代码(展开查看)
#include "threadpool.h" #include <iostream> void fun1(int slp) { printf(" hello, fun1 ! %dn" ,std::this_thread::get_id()); if (slp>0) { printf(" ======= fun1 sleep %d ========= %dn",slp, std::this_thread::get_id()); std::this_thread::sleep_for(std::chrono::milliseconds(slp)); } } struct gfun { int operator()(int n) { printf("%d hello, gfun ! %dn" ,n, std::this_thread::get_id() ); return 42; } }; class A { public: static int Afun(int n = 0) { //函数必须是 static 的才能直接使用线程池 std::cout << n << " hello, Afun ! " << std::this_thread::get_id() << std::endl; return n; } static std::string Bfun(int n, std::string str, char c) { std::cout << n << " hello, Bfun ! "<< str.c_str() <<" " << (int)c <<" " << std::this_thread::get_id() << std::endl; return str; } }; int main() try { std::threadpool executor{ 50 }; A a; std::future<void> ff = executor.commit(fun1,0); std::future<int> fg = executor.commit(gfun{},0); std::future<int> gg = executor.commit(a.Afun, 9999); //IDE提示错误,但可以编译运行 std::future<std::string> gh = executor.commit(A::Bfun, 9998,"mult args", 123); std::future<std::string> fh = executor.commit([]()->std::string { std::cout << "hello, fh ! " << std::this_thread::get_id() << std::endl; return "hello,fh ret !"; }); std::cout << " ======= sleep ========= " << std::this_thread::get_id() << std::endl; std::this_thread::sleep_for(std::chrono::microseconds(900)); for (int i = 0; i < 50; i++) { executor.commit(fun1,i*100 ); } std::cout << " ======= commit all ========= " << std::this_thread::get_id()<< " idlsize="<<executor.idlCount() << std::endl; std::cout << " ======= sleep ========= " << std::this_thread::get_id() << std::endl; std::this_thread::sleep_for(std::chrono::seconds(3)); ff.get(); //调用.get()获取返回值会等待线程执行完,获取返回值 std::cout << fg.get() << " " << fh.get().c_str()<< " " << std::this_thread::get_id() << std::endl; std::cout << " ======= sleep ========= " << std::this_thread::get_id() << std::endl; std::this_thread::sleep_for(std::chrono::seconds(3)); std::cout << " ======= fun1,55 ========= " << std::this_thread::get_id() << std::endl; executor.commit(fun1,55).get(); //调用.get()获取返回值会等待线程执行完 std::cout << "end... " << std::this_thread::get_id() << std::endl; std::threadpool pool(4); std::vector< std::future<int> > results; for (int i = 0; i < 8; ++i) { results.emplace_back( pool.commit([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; }) ); } std::cout << " ======= commit all2 ========= " << std::this_thread::get_id() << std::endl; for (auto && result : results) std::cout << result.get() << ' '; std::cout << std::endl; return 0; } catch (std::exception& e) { std::cout << "some unhappy happened... " << std::this_thread::get_id() << e.what() << std::endl; }
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