其实关于C++线程池的文章我好久以前写过，但估计很多新朋友都没有看到过，这里也重新发一下！
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：
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，is_shutdown_now置为true表示立即关闭线程，is_shutdown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is_shutdown_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(); }
waiting_thread_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(); }
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;
}关于线程池个人认为还应该有定时器功能和循环执行某个任务的功能，这两个功能我是单独封装成一个类实现 。
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