大家好，我是程序员喵大人，今天我们来讲下C++线程池的实现。
本人在开发过程中经常会遇到需要使用线程池的需求，但查了一圈发现在C++中完备的线程池第三方库还是比较少的，于是打算自己搞一个，链接地址文章最后附上，目前还只是初版，可能还有很多问题，望各位指正。
个人认为线程池需要支持以下几个基本功能：
如何实现线程池？下面是自己实现的线程池逻辑。
1. 链表或者数组：用于存储线程池中的线程。
2. 队列：用于存储需要放入线程池中执行的任务。
3. 条件变量：当有任务需要执行时，用于通知正在等待的线程从任务队列中取出任务执行。
代码如下：
class ThreadPool {
public:
using PoolSeconds = std::chrono::seconds;
/** 线程池的配置
* core_threads: 核心线程个数，线程池中最少拥有的线程个数，初始化就会创建好的线程，常驻于线程池
*
* max_threads: >=core_threads，当任务的个数太多线程池执行不过来时，
* 内部就会创建更多的线程用于执行更多的任务，内部线程数不会超过max_threads
*
* max_task_size: 内部允许存储的最大任务个数，暂时没有使用
*
* time_out: Cache线程的超时时间，Cache线程指的是max_threads-core_threads的线程,
* 当time_out时间内没有执行任务，此线程就会被自动回收
*/
struct ThreadPoolConfig {
int core_threads;
int max_threads;
int max_task_size;
PoolSeconds time_out;
};
/**
* 线程的状态：有等待、运行、停止
*/
enum class ThreadState { kInit = 0, kWaiting = 1, kRunning = 2, kStop = 3 };
/**
* 线程的种类标识：标志该线程是核心线程还是Cache线程，Cache是内部为了执行更多任务临时创建出来的
*/
enum class ThreadFlag { kInit = 0, kCore = 1, kCache = 2 };
using ThreadPtr = std::shared_ptr<std::thread>;
using ThreadId = std::atomic<int>;
using ThreadStateAtomic = std::atomic<ThreadState>;
using ThreadFlagAtomic = std::atomic<ThreadFlag>;
/**
* 线程池中线程存在的基本单位，每个线程都有个自定义的ID，有线程种类标识和状态
*/
struct ThreadWrapper {
ThreadPtr ptr;
ThreadId id;
ThreadFlagAtomic flag;
ThreadStateAtomic state;
ThreadWrapper() {
ptr = nullptr;
id = 0;
state.store(ThreadState::kInit);
}
};
using ThreadWrapperPtr = std::shared_ptr<ThreadWrapper>;
using ThreadPoolLock = std::unique_lock<std::mutex>;
private:
ThreadPoolConfig config_;
std::list<ThreadWrapperPtr> worker_threads_;
std::queue<std::function<void()>> tasks_;
std::mutex task_mutex_;
std::condition_variable task_cv_;
std::atomic<int> total_function_num_;
std::atomic<int> waiting_thread_num_;
std::atomic<int> thread_id_; // 用于为新创建的线程分配ID
std::atomic<bool> is_shutdown_now_;
std::atomic<bool> is_shutdown_;
std::atomic<bool> is_available_;
};在构造函数中将各个成员变量都附初值，同时判断线程池的config是否合法。
ThreadPool(ThreadPoolConfig config) : config_(config) {
this->total_function_num_.store(0);
this->waiting_thread_num_.store(0);
this->thread_id_.store(0);
this->is_shutdown_.store(false);
this->is_shutdown_now_.store(false);
if (IsValidConfig(config_)) {
is_available_.store(true);
} else {
is_available_.store(false);
}
}
bool IsValidConfig(ThreadPoolConfig config) {
if (config.core_threads < 1 || config.max_threads < config.core_threads || config.time_out.count() < 1) {
return false;
}
return true;
}创建核心线程数个线程，常驻于线程池，等待任务的执行，线程ID由GetNextThreadId()统一分配。
// 开启线程池功能
bool Start() {
if (!IsAvailable()) {
return false;
}
int core_thread_num = config_.core_threads;
cout << "Init thread num " << core_thread_num << endl;
while (core_thread_num-- > 0) {
AddThread(GetNextThreadId());
}
cout << "Init thread end" << endl;
return true;
}这里有两个标志位，isshutdown_now置为true表示立即关闭线程，isshutdown置为true则表示先执行完队列里的任务再关闭线程池。
// 关掉线程池，内部还没有执行的任务会继续执行
void ShutDown() {
ShutDown(false);
cout << "shutdown" << endl;
}
// 执行关掉线程池，内部还没有执行的任务直接取消，不会再执行
void ShutDownNow() {
ShutDown(true);
cout << "shutdown now" << endl;
}
// private
void ShutDown(bool is_now) {
if (is_available_.load()) {
if (is_now) {
this->is_shutdown_now_.store(true);
} else {
this->is_shutdown_.store(true);
}
this->task_cv_.notify_all();
is_available_.store(false);
}
}见AddThread()函数，默认会创建Core线程，也可以选择创建Cache线程，线程内部会有一个死循环，不停的等待任务，有任务到来时就会执行，同时内部会判断是否是Cache线程，如果是Cache线程，timeout时间内没有任务执行就会自动退出循环，线程结束。
这里还会检查is_shutdown和isshutdown_now标志，根据两个标志位是否为true来判断是否结束线程。
void AddThread(int id) { AddThread(id, ThreadFlag::kCore); }
void AddThread(int id, ThreadFlag thread_flag) {
cout << "AddThread " << id << " flag " << static_cast<int>(thread_flag) << endl;
ThreadWrapperPtr thread_ptr = std::make_shared<ThreadWrapper>();
thread_ptr->id.store(id);
thread_ptr->flag.store(thread_flag);
auto func = [this, thread_ptr]() {
for (;;) {
std::function<void()> task;
{
ThreadPoolLock lock(this->task_mutex_);
if (thread_ptr->state.load() == ThreadState::kStop) {
break;
}
cout << "thread id " << thread_ptr->id.load() << " running start" << endl;
thread_ptr->state.store(ThreadState::kWaiting);
++this->waiting_thread_num_;
bool is_timeout = false;
if (thread_ptr->flag.load() == ThreadFlag::kCore) {
this->task_cv_.wait(lock, [this, thread_ptr] {
return (this->is_shutdown_ || this->is_shutdown_now_ || !this->tasks_.empty() ||
thread_ptr->state.load() == ThreadState::kStop);
});
} else {
this->task_cv_.wait_for(lock, this->config_.time_out, [this, thread_ptr] {
return (this->is_shutdown_ || this->is_shutdown_now_ || !this->tasks_.empty() ||
thread_ptr->state.load() == ThreadState::kStop);
});
is_timeout = !(this->is_shutdown_ || this->is_shutdown_now_ || !this->tasks_.empty() ||
thread_ptr->state.load() == ThreadState::kStop);
}
--this->waiting_thread_num_;
cout << "thread id " << thread_ptr->id.load() << " running wait end" << endl;
if (is_timeout) {
thread_ptr->state.store(ThreadState::kStop);
}
if (thread_ptr->state.load() == ThreadState::kStop) {
cout << "thread id " << thread_ptr->id.load() << " state stop" << endl;
break;
}
if (this->is_shutdown_ && this->tasks_.empty()) {
cout << "thread id " << thread_ptr->id.load() << " shutdown" << endl;
break;
}
if (this->is_shutdown_now_) {
cout << "thread id " << thread_ptr->id.load() << " shutdown now" << endl;
break;
}
thread_ptr->state.store(ThreadState::kRunning);
task = std::move(this->tasks_.front());
this->tasks_.pop();
}
task();
}
cout << "thread id " << thread_ptr->id.load() << " running end" << endl;
};
thread_ptr->ptr = std::make_shared<std::thread>(std::move(func));
if (thread_ptr->ptr->joinable()) {
thread_ptr->ptr->detach();
}
this->worker_threads_.emplace_back(std::move(thread_ptr));
}见如下代码，将任务使用std::bind封装成std::function放入任务队列中，任务较多时内部还会判断是否有空闲线程，如果没有空闲线程，会自动创建出最多(max_threads-core_threads)个Cache线程用于执行任务。
// 放在线程池中执行函数
template <typename F, typename... Args>
auto Run(F &&f, Args &&... args) -> std::shared_ptr<std::future<std::result_of_t<F(Args...)>>> {
if (this->is_shutdown_.load() || this->is_shutdown_now_.load() || !IsAvailable()) {
return nullptr;
}
if (GetWaitingThreadSize() == 0 && GetTotalThreadSize() < config_.max_threads) {
AddThread(GetNextThreadId(), ThreadFlag::kCache);
}
using return_type = std::result_of_t<F(Args...)>;
auto task = std::make_shared<std::packaged_task<return_type()>>(
std::bind(std::forward<F>(f), std::forward<Args>(args)...));
total_function_num_++;
std::future<return_type> res = task->get_future();
{
ThreadPoolLock lock(this->task_mutex_);
this->tasks_.emplace([task]() { (*task)(); });
}
this->task_cv_.notify_one();
return std::make_shared<std::future<std::result_of_t<F(Args...)>>>(std::move(res));
}int GetTotalThreadSize() { return this->worker_threads_.size(); }waitingthread_num值表示空闲线程的个数，该变量在线程循环内部会更新。
int GetWaitingThreadSize() { return this->waiting_thread_num_.load(); }简单的测试代码
int main() {
cout << "hello" << endl;
ThreadPool pool(ThreadPool::ThreadPoolConfig{4, 5, 6, std::chrono::seconds(4)});
pool.Start();
std::this_thread::sleep_for(std::chrono::seconds(4));
cout << "thread size " << pool.GetTotalThreadSize() << endl;
std::atomic<int> index;
index.store(0);
std::thread t([&]() {
for (int i = 0; i < 10; ++i) {
pool.Run([&]() {
cout << "function " << index.load() << endl;
std::this_thread::sleep_for(std::chrono::seconds(4));
index++;
});
// std::this_thread::sleep_for(std::chrono::seconds(2));
}
});
t.detach();
cout << "=================" << endl;
std::this_thread::sleep_for(std::chrono::seconds(4));
pool.Reset(ThreadPool::ThreadPoolConfig{4, 4, 6, std::chrono::seconds(4)});
std::this_thread::sleep_for(std::chrono::seconds(4));
cout << "thread size " << pool.GetTotalThreadSize() << endl;
cout << "waiting size " << pool.GetWaitingThreadSize() << endl;
cout << "---------------" << endl;
pool.ShutDownNow();
getchar();
cout << "world" << endl;
return 0;
}Copyright© 2013-2020
All Rights Reserved 京ICP备2023019179号-8