adding the functor program

lectures/21_hash_tables_II/functor.cpp

@@ -0,0 +1,17 @@
+#include <iostream>
+
+class MyFunctor {
+public:
+    int operator()(int x, int y) {
+        return x + y;
+    }
+};
+
+int main() {
+    MyFunctor myFunc;
+    int result = myFunc(3, 4);  // This calls the overloaded () operator.
+    // result now holds the value 7.
+    std::cout << "result is " << result << std::endl;
+    return 0;
+}
+