Functions allow you to segment your code into logical routines, which makes your code more readable, and gives the flexibility of being able to reuse code. C++ provides a wealth of options to define functions, including variable argument lists, templates, function pointers, and lambda expressions. However, there is one main issue with global functions: the data is separate from the function. This means that the function has to access the data via global data items, or data has to be passed to a function via a parameter every time the function is called. In both cases, the data exists outside the function and could be used by other functions unrelated to the data. The next chapter will give a solution to this: classes. A class allows you to encapsulate data in a custom type, and you can define functions on that type so that only these functions will be able to access the data.