add virtual methods

add interface
add pure virtual
add casts
add template example

CPlusPlus/intro/ClassA.h

@@ -1,6 +1,7 @@
 #pragma once
+#include "IPrintable.h"
 
-class ClassA
+class ClassA : public IPrintable
 {
 	// private
 
@@ -10,6 +11,9 @@ public:
 	ClassA(const ClassA& other); // copy constructor
 	~ClassA(); // destructor
 
+	const int& GetNumber() const { return *_pNumber; } // implicit inline method
+	void Print() override;
+
 protected:
 	// protected