Add example of visitor pattern in C++

cpp/patterns/visitor/main.cpp

@@ -0,0 +1,16 @@
+
+#include <iostream>
+
+#include "visitor.hpp"
+
+int main(const int argc, const char * argv[])
+{
+  Gear *newGear = new Gear(1.5f, 1.0f);
+  PartVisitor *newVisitor = new PartVisitor;
+  // Test visiting the Gear concrete component with PartVisitor
+  newGear->accept(newVisitor);
+
+  // Testing a Spring concrete component on the same visitor
+  auto *newSpring = new Spring(2.5f, 5.0f);
+  newSpring->accept(newVisitor);
+}