Use explicit constructor to avoid implicit conversions

Author: jhasse

ThreadPool.hpp

@@ -7,7 +7,7 @@
 
 class ThreadPool {
 public:
-    ThreadPool(size_t);
+    explicit ThreadPool(size_t);
     template<class F, class... Args>
     auto enqueue(F&& f, Args&&... args)
         -> std::future<std::invoke_result_t<F, Args...>>;