added getter for the number of workers and default constructor

ThreadPool.h

@@ -10,13 +10,16 @@
 #include <future>
 #include <functional>
 #include <stdexcept>
+#include <algorithm>
 
 class ThreadPool {
 public:
     ThreadPool(size_t);
+    ThreadPool();
     template<class F, class... Args>
     auto enqueue(F&& f, Args&&... args) 
         -> std::future<typename std::result_of<F(Args...)>::type>;
+    size_t noWorkers() const;
     ~ThreadPool();
 private:
     // need to keep track of threads so we can join them
@@ -54,6 +57,11 @@ inline ThreadPool::ThreadPool(size_t threads)
         );
 }
 
+// the default number of workers is given by max(1, std::thread::hardware_concurrency())
+inline ThreadPool::ThreadPool() : ThreadPool(std::max(1u, std::thread::hardware_concurrency()))
+{
+}
+
 // add new work item to the pool
 template<class F, class... Args>
 auto ThreadPool::enqueue(F&& f, Args&&... args) 
@@ -78,6 +86,12 @@ auto ThreadPool::enqueue(F&& f, Args&&... args)
     return res;
 }
 
+// it may be useful to retrieve the number of workers at runtime
+inline size_t ThreadPool::noWorkers() const
+{
+    return workers.size();
+}
+
 // the destructor joins all threads
 inline ThreadPool::~ThreadPool()
 {

example.cpp

@@ -7,8 +7,10 @@
 int main()
 {
 
-    ThreadPool pool(4);
+    ThreadPool pool;
     std::vector< std::future<int> > results;
+
+    std::cout << "Number of workers: " << pool.noWorkers() << std::endl;
 
     for(int i = 0; i < 8; ++i) {
         results.push_back(