Add static vs non-static | non-stateful vs stateful

CM2005 Object Oriented Programming/Topic 3/3.5.1/static.cpp

@@ -0,0 +1,12 @@
+#include <string>
+#include <iostream>
+
+int main()
+{
+  std::string s = "Matthew";
+  std::cout << "Length of the string is " << s.length() << std::endl; // .length is stateful
+  // the life of a stateful function ends with the object that it belongs to
+
+  // static or non-stateful function, they use memory and then return it
+  std::cout << std::stod("100") << std::endl;
+}