Encapsulation

Encapsulation is the grouping together of data and logic into a single unit. In object-oriented programming, classes encapsulate data and functions that operate on that data.

This can be a delicate balance, because on the one hand we want to group together relevant data and functions, but on the hand we want to limit member functions to only those functions that need direct access to the representation of a class.

In the context of a Date class, a function Date Tomorrow(Date const & date) probably does not need to be encapsulated as a class member. It can exist outside the Date class.

However, a function that calculates the number of days in a month probably should be encapsulated with the class, because the class needs this function in order to operate correctly.