added some comments to explain the code

Function.hpp

@@ -3,15 +3,15 @@
 
 
 
-template <typename Fun>
+template <typename Fun>     // here Fun will be a function signature
 class Function;
 
 
-template <typename Ret, typename... Args>
+template <typename Ret, typename... Args>       // specialization, so we can work
 class Function<Ret(Args...)>
 {
 private:
-    class callable_base
+    class callable_base     // type-erasure magic for functors
     {
     public:
         virtual Ret call(Args...) const = 0;
@@ -52,7 +52,7 @@ public:
         return *this;
     }
 
-    Function& operator=(std::nullptr_t)
+    Function& operator=(std::nullptr_t)     // exploiting that decltype(nullptr) is std::nullptr_t
     { _is_null = true; }
 
     Ret operator()(Args... args) const