Added class and function docs for thread pool

include/cpr/threadpool.h

@@ -13,35 +13,111 @@
 #include <thread>
 
 namespace cpr {
+/**
+ * cpr thread pool implementation used by async requests.
+ *
+ * Example:
+ * // Create a new thread pool object
+ * cpr::ThreadPool tp;
+ * // Start the thread pool and spawn initial set of worker threads.
+ * tp.Start()
+ * // Add work
+ * tp.Submit(..)
+ * ...
+ * // Stop/join workers and flush the task queue
+ * tp.Stop()
+ * // Start the thread pool again spawning the initial set of worker threads.
+ * tp.Start()
+ * ...
+ **/
 class ThreadPool {
   public:
+    /**
+     * The default minimum thread count for the thread pool.
+     * Even if there is no work this number of threads should be in standby for once new work arrives.
+     **/
     static constexpr size_t DEFAULT_MIN_THREAD_COUNT = 0;
+    /**
+     * The default maximum thread count for the thread pool.
+     * Even if there is a lot of work, the thread pool is not allowed to create more threads than this number.
+     **/
     static size_t DEFAULT_MAX_THREAD_COUNT;
 
   private:
+    /**
+     * The thread pool or worker thread state.
+     **/
     enum class State : uint8_t { STOP, RUNNING };
+    /**
+     * Collection of properties identifying a worker thread for the thread pool.
+     **/
     struct WorkerThread {
         std::unique_ptr<std::thread> thread{nullptr};
+        /**
+         * RUNNING: The thread is still active and working on or awaiting new work.
+         * STOP: The thread is shutting down or has already been shut down and is ready to be joined.
+         **/
         State state{State::RUNNING};
     };
 
+    /**
+     * Mutex for synchronizing access to the worker thread list.
+     **/
     std::mutex workerMutex;
+    /**
+     * A list of all worker threads
+     **/
     std::list<WorkerThread> workers;
+    /**
+     * Number of threads ready to be joined where their state is 'STOP'.
+     **/
     std::atomic_size_t workerJoinReadyCount{0};
 
+    /**
+     * Mutex for synchronizing access to the task queue.
+     **/
     std::mutex taskQueueMutex;
+    /**
+     * Conditional variable to let threads wait for new work to arrive.
+     **/
     std::condition_variable taskQueueCondVar;
+    /**
+     * A queue of tasks synchronized by 'taskQueueMutex'.
+     **/
     std::queue<std::function<void()>> tasks;
 
-    std::atomic<State> state = State::STOP;
+    /**
+     * The current state for the thread pool.
+     **/
+    std::atomic<State> state = State::RUNNING;
+    /**
+     * The number of threads that should always be in standby or working.
+     **/
     std::atomic_size_t minThreadCount;
+    /**
+     * The current number of threads available to the thread pool (working or idle).
+     **/
     std::atomic_size_t curThreadCount{0};
+    /**
+     * The maximum number of threads allowed to be used by this thread pool.
+     **/
     std::atomic_size_t maxThreadCount;
+    /**
+     * The number of idle threads without any work awaiting new tasks.
+     **/
     std::atomic_size_t idleThreadCount{0};
 
+    /**
+     * General control mutex synchronizing access to internal thread pool resources.
+     **/
     std::recursive_mutex controlMutex;
 
   public:
+    /**
+     * Creates a new thread pool object with a minimum and maximum thread count.
+     * minThreadCount: Number of threads that should always be in standby or working.
+     * maxThreadCount: The maximum number of threads allowed to be used by this thread pool.
+     **/
     explicit ThreadPool(size_t minThreadCount = DEFAULT_MIN_THREAD_COUNT, size_t maxThreadCount = DEFAULT_MAX_THREAD_COUNT);
     ThreadPool(const ThreadPool& other) = delete;
     ThreadPool(ThreadPool&& old) = delete;
@@ -50,24 +126,60 @@ class ThreadPool {
     ThreadPool& operator=(const ThreadPool& other) = delete;
     ThreadPool& operator=(ThreadPool&& old) = delete;
 
+    /**
+     * Returns the current thread pool state.
+     * The thread pool is in STOP state when initially created and will move over to RUNNING once Start() is invoked for the first time.
+     **/
     [[nodiscard]] State GetState() const;
+    /**
+     * Returns the maximum number of threads allowed to be used by this thread pool.
+     **/
     [[nodiscard]] size_t GetMaxThreadCount() const;
+    /**
+     * Returns the current number of threads available to the thread pool (working or idle).
+     **/
     [[nodiscard]] size_t GetCurThreadCount() const;
+    /**
+     * Returns the number of idle threads without any work awaiting new tasks.
+     **/
     [[nodiscard]] size_t GetIdleThreadCount() const;
+    /**
+     * Returns the number of threads that should always be in standby or working.
+     **/
     [[nodiscard]] size_t GetMinThreadCount() const;
 
+    /**
+     * Sets the number of threads that should always be in standby or working.
+     **/
     void SetMinThreadCount(size_t minThreadCount);
+    /**
+     * Sets the current number of threads available to the thread pool (working or idle).
+     **/
     void SetMaxThreadCount(size_t maxThreadCount);
 
+    /**
+     * Starts the thread pool by spawning GetMinThreadCount() threads.
+     * Does nothing in case the thread pool state is already RUNNING.
+     **/
     void Start();
+    /**
+     * Sets the state to STOP, clears the task queue and joins all running threads.
+     * This means waiting for all threads that currently work on something letting them finish their task.
+     **/
     void Stop();
+    /**
+     * Returns as soon as the task queue is empty and all threads are either stopped/joined or in idel state.
+     **/
     void Wait();
 
     /**
+     * Enqueues a new task to the thread pool.
      * Return a future, calling future.get() will wait task done and return RetType.
      * Submit(fn, args...)
      * Submit(std::bind(&Class::mem_fn, &obj))
      * Submit(std::mem_fn(&Class::mem_fn, &obj))
+     *
+     * Will start a new thread in case all other threads are currently working and GetCurThreadCount() < GetMaxThreadCount().
      **/
     template <class Fn, class... Args>
     auto Submit(Fn&& fn, Args&&... args) {
@@ -96,10 +208,23 @@ class ThreadPool {
     }
 
   private:
+    /**
+     * Sets the new thread pool state.
+     * Returns true in case the current state was different to the newState.
+     **/
     bool setState(State newState);
+    /**
+     * Adds a new worker thread.
+     **/
     void addThread();
+    /**
+     * Goes through the worker threads list and joins all threads where their state is STOP.
+     **/
     void joinStoppedThreads();
 
+    /**
+     * The thread entry point where the heavy lifting happens.
+     **/
     void threadFunc(WorkerThread& workerThread);
 };
 } // namespace cpr