What are the access levels allowed by default access modifier (i.e. no access modifier)?

A class member with no access modifier specified can be accessed by any class in the same package. It need not be a derived class of the containing class. These members, though, are not visible outside the package of the containing class.

What is a virtual memory?

Primary memory (RAM) of a computer is limited. Many computer programs require more memory than is limited by the primary memory. Virtual memory is used in a computer to act as a primary memory, which a computer program can access. It is nothing but a part of secondary storage, which a computer program views as primary storage.

What is the difference between pass by value and pass by reference?

Parameters to a C++ function can either be passed by value or by reference. When passed by reference, the original variable which is passed as an argument is used throughout the function. Thus, any change made to the argument in the function will persist after the function ends When passed by value, a copy of the argument variable is passed to the function. Thus, any change to the argument will not reflect in the original argument variable.