add virtual methods

add interface
add pure virtual
add casts
add template example

CPlusPlus/intro/IPrintable.h

@@ -0,0 +1,10 @@
+#pragma once
+#include <iostream>
+
+class IPrintable
+{
+public:
+	IPrintable() { std::cout << "IPrintable called" << std::endl; }
+	virtual ~IPrintable() { std::cout << "~IPrintable called" << std::endl; }
+	virtual void Print() = 0; // pure virtual
+};