add: exercise answer for chapter 6 and 7

exercises/7/ThreadPool.hpp

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+#ifndef THREAD_POOL_H
+#define THREAD_POOL_H
+
+#include <vector>               // std::vector
+#include <queue>                // std::queue
+#include <memory>               // std::make_shared
+
+#include <thread>               // std::thread
+#include <mutex>                // std::mutex, std::unique_lock
+#include <condition_variable>   // std::condition_variable
+#include <future>               // std::future, std::packaged_task
+
+#include <functional>           // std::function, std::bind
+#include <stdexcept>            // std::runtime_error
+#include <utility>              // std::move, std::forward
+
+class ThreadPool {
+public:
+    
+    // initialize the number of concurrency threads
+    ThreadPool(size_t);
+    
+    // enqueue new thread task
+    template<class F, class... Args>
+    auto enqueue(F&& f, Args&&... args) 
+        -> std::future<typename std::result_of<F(Args...)>::type>;
+    
+    // destroy thread pool and all created threads
+    ~ThreadPool();
+private:
+    
+    // thread list, stores all threads
+    std::vector< std::thread > workers;
+    // queue task, the type of queue elements are functions with void return type
+    std::queue< std::function<void()> > tasks;
+    
+    // for synchonization
+    std::mutex queue_mutex;
+    // std::condition_variable is a new feature from c++11,
+    // it's a synchronization primitives. it can be used 
+    // to block a thread or threads at the same time until
+    // all of them modified condition_variable.
+    std::condition_variable condition;
+    bool stop;
+};
+ 
+// constructor initialize a fixed size of worker
+inline ThreadPool::ThreadPool(size_t threads): stop(false)
+{
+    // initialize worker
+    for(size_t i = 0;i<threads;++i)
+        // std::vector::emplace_back :
+        //    append to the end of vector container
+        //    this element will be constructed at the end of container, without copy and move behavior
+        workers.emplace_back(
+            // the lambda express capture this, i.e. the instance of thread pool
+            [this]
+            {
+                // avoid fake awake
+                for(;;)
+                {
+                    
+                    // define function task container, return type is void
+                    std::function<void()> task;
+
+                    // critical section
+                    {
+                        // get mutex
+                        std::unique_lock<std::mutex> lock(this->queue_mutex);
+                        
+                        // block current thread
+                        this->condition.wait(lock,
+                            [this]{ return this->stop || !this->tasks.empty(); });
+                        
+                        // return if queue empty and task finished
+                        if(this->stop && this->tasks.empty())
+                            return;
+                        
+                        // otherwise execute the first element of queue
+                        task = std::move(this->tasks.front());
+                        this->tasks.pop();
+                    }
+                    
+                    // execution
+                    task();
+                }
+            }
+        );
+}
+
+// Enqueue a new thread
+// use variadic templates and tail return type 
+template<class F, class... Args>
+auto ThreadPool::enqueue(F&& f, Args&&... args) 
+    -> std::future<typename std::result_of<F(Args...)>::type>
+{
+    // deduce return type
+    using return_type = typename std::result_of<F(Args...)>::type;
+    
+    // fetch task
+    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();
+    
+    // critical section
+    {
+        std::unique_lock<std::mutex> lock(queue_mutex);
+
+        // avoid add new thread if theadpool is destroied
+        if(stop)
+            throw std::runtime_error("enqueue on stopped ThreadPool");
+
+        // add thread to queue
+        tasks.emplace([task]{ (*task)(); });
+    }
+    
+    // notify a wait thread
+    condition.notify_one();
+    return res;
+}
+
+// destroy everything
+inline ThreadPool::~ThreadPool()
+{
+    // critical section
+    {
+        std::unique_lock<std::mutex> lock(queue_mutex);
+        stop = true;
+    }
+    
+    // wake up all threads
+    condition.notify_all();
+    
+    // let all processes into synchronous execution, use c++11 new for-loop: for(value:values)
+    for(std::thread &worker: workers)
+        worker.join();
+}
+
+#endif

exercises/7/main.cpp

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+#include <iostream> // std::cout, std::endl
+
+#include <vector>   // std::vector
+#include <string>   // std::string
+#include <future>   // std::future
+#include <thread>   // std::this_thread::sleep_for
+#include <chrono>   // std::chrono::seconds
+
+#include "ThreadPool.hpp"
+
+int main()
+{
+    // create a thread pool with max. 4 concurrency threads
+    ThreadPool pool(4);
+    // create execution results list
+    std::vector< std::future<std::string> > results;
+
+    // start eight thread task
+    for(int i = 0; i < 8; ++i) {
+        // add all task to result list
+        results.emplace_back(
+            // ass print task to thread pool
+            pool.enqueue([i] {
+                std::cout << "hello " << i << std::endl;
+                // wait a sec when the previous line is out
+                std::this_thread::sleep_for(std::chrono::seconds(1));
+                // keep output and return the status of execution
+                std::cout << "world " << i << std::endl;
+                return std::string("---thread ") + std::to_string(i) + std::string(" finished.---");
+            })
+        );
+    }
+
+    // outputs
+    for(auto && result: results)
+        std::cout << result.get() << ' ';
+    std::cout << std::endl;
+    
+    return 0;
+}