When to use functions

Functions encapsulate a unit of computation and are most often used to allow that unit of computation to be used in many places. In their pure form, they operate on one or more input values to produce one or more output values. (Of course, C++ functions can only have a single return value, but we’ll come back to this.) The term “pure” means that the function itself is independent of the global program state; only the input data has any effect on the outputs. Non-pure functions have their uses too, but are far less easy to reason about. For this reason, we shall mostly restrict our attention to pure functions here.

Pure functions are a mathematical concept, defined as a relation between two sets under which each member of the “input” set is related to exactly one element of the “output” set (the codomain). That is, any given configuration of inputs should always produce the same output. This is obviously a very...