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/******************************************************************************
*
* Copyright (C) 1997-2020 by Dimitri van Heesch.
*
* Permission to use, copy, modify, and distribute this software and its
* documentation under the terms of the GNU General Public License is hereby
* granted. No representations are made about the suitability of this software
* for any purpose. It is provided "as is" without express or implied warranty.
* See the GNU General Public License for more details.
*
* Documents produced by Doxygen are derivative works derived from the
* input used in their production; they are not affected by this license.
*
*/
#ifndef THREADPOOL_H
#define THREADPOOL_H
#include <condition_variable>
#include <deque>
#include <functional>
#include <future>
#include <mutex>
#include <thread>
#include <type_traits>
#include <utility>
#include <vector>
/// Class managing a pool of worker threads.
/// Work can be queued by passing a function to queue(). A future will be
/// returned that can be used to obtain the result of the function after execution.
///
/// Usage example:
/// @code
/// ThreadPool pool(10);
/// std::vector< std::future< int > > results;
/// for (int i=0;i<10;i++)
/// {
/// auto run = [](int i) { return i*i; };
/// results.emplace_back(pool.queue(std::bind(run,i)));
/// }
/// for (auto &f : results)
/// {
/// printf("Result %d:\n", f.get());
/// }
/// @endcode
class ThreadPool
{
public:
/// start N threads in the thread pool.
ThreadPool(std::size_t N=1)
{
for (std::size_t i = 0; i < N; ++i)
{
// each thread is a std::async running thread_task():
m_finished.push_back(
std::async(
std::launch::async,
[this]{ threadTask(); }
)
);
}
}
/// deletes the thread pool by finishing all threads
~ThreadPool()
{
finish();
}
/// Queue the callable function \a f for the threads to execute.
/// A future of the return type of the function is returned to capture the result.
template<class F, class R=std::result_of_t<F&()> >
std::future<R> queue(F&& f)
{
// We wrap the function object into a packaged task, splitting
// execution from the return value.
// Since the packaged_task object is not copyable, we create it on the heap
// and capture it via a shared pointer in a lambda and then assign that lambda
// to a std::function.
auto ptr = std::make_shared< std::packaged_task<R()> >(std::forward<F>(f));
auto taskFunc = [ptr]() { if (ptr->valid()) (*ptr)(); };
auto r=ptr->get_future(); // get the return value before we hand off the task
{
std::unique_lock<std::mutex> l(m_mutex);
m_work.emplace_back(taskFunc);
m_cond.notify_one(); // wake a thread to work on the task
}
return r; // return the future result of the task
}
/// finish enques a "stop the thread" message for every thread,
/// then waits for them to finish
void finish()
{
{
std::unique_lock<std::mutex> l(m_mutex);
for(auto&& u : m_finished)
{
unused_variable(u);
m_work.push_back({}); // insert empty function object to signal abort
}
}
m_cond.notify_all();
m_finished.clear();
}
private:
// helper to silence the compiler warning about unused variables
template <typename ...Args>
void unused_variable(Args&& ...args) { (void)(sizeof...(args)); }
// the work that a worker thread does:
void threadTask()
{
while(true)
{
// pop a task off the queue:
std::function<void()> f;
{
// usual thread-safe queue code:
std::unique_lock<std::mutex> l(m_mutex);
if (m_work.empty())
{
m_cond.wait(l,[&]{return !m_work.empty();});
}
f = std::move(m_work.front());
m_work.pop_front();
}
// if the function is empty, it means we are asked to abort
if (!f) return;
// run the task
f();
}
}
// the mutex, condition variable and deque form a single
// thread-safe triggered queue of tasks:
std::mutex m_mutex;
std::condition_variable m_cond;
// hold the queue of work
std::deque< std::function<void()> > m_work;
// this holds futures representing the worker threads being done:
std::vector< std::future<void> > m_finished;
};
#endif
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