C++ specific Concepts

static:
http://www.cprogramming.com/tutorial/statickeyword.html
http://quiz.geeksforgeeks.org/c-static-keyword-question-1/
http://quiz.geeksforgeeks.org/c-plus-plus/static-keyword/

friend:
http://www.cprogramming.com/tutorial/friends.html
http://www.tutorialspoint.com/cplusplus/cpp_friend_functions.htm
http://quiz.geeksforgeeks.org/c-plus-plus/friend-function-and-class/

Const:
http://www.studytonight.com/cpp/const-keyword.php

namespace:
http://www.cplusplus.com/doc/tutorial/namespaces/

virtual function:
http://stackoverflow.com/questions/2391679/why-do-we-need-virtual-functions-in-c?rq=1
http://www.studytonight.com/cpp/virtual-functions.php
http://www.programcreek.com/2011/01/a-simple-example-of-c-virtual-keyword/
http://quiz.geeksforgeeks.org/c-plus-plus/virtual-functions/

Copy Constructor:
http://www.studytonight.com/cpp/copy-constructor-in-cpp.php

Reference types in C++ vs Pointers in C:
http://www.tutorialspoint.com/cplusplus/cpp_references.htm
http://www.cprogramming.com/tutorial/references.html

Interface vs Abstract Class:
http://www.brushupmyskill.com/2016/01/differences-between-interface-and.html