adding the multiply example

lectures/14_stacks_queues/multiply.cpp

@@ -0,0 +1,26 @@
+#include <iostream>
+
+// functor class
+class MultiplyBy {
+private:
+	int factor;
+
+public:
+	// constructor
+	MultiplyBy(int factor) : factor(factor) {}
+
+	// overloaded function call operator
+	int operator()(int x) const {
+		return x * factor;
+	}
+};
+
+int main() {
+	// create an instance of the functor
+	MultiplyBy multiplyByTwo(2);
+
+	// use the functor as a function
+	std::cout << "Result of multiplying 5 by 2: " << multiplyByTwo(5) << std::endl;
+
+	return 0;
+}