add forward declaration examples

add classes

CPlusPlus/Classes/main.cpp

@@ -0,0 +1,22 @@
+#include <iostream>
+#include "Entity.h"
+#include "Monster.h"
+#include "Vector3.h"
+
+using namespace MyNamespace;
+using namespace std;
+
+int main()
+{
+	Entity entity; // this is an instance
+	cout << entity.ToString() << endl;
+
+	Monster monster;
+	cout << monster.ToString() << endl;
+
+	Vector3 position(1.0f, 0.0f, 2.0f);
+	Vector3 position2 = Vector3();
+	Vector3 position3{}; // list-initializer
+
+	return 0;
+}