Summary

Predicate purity: A predicate is a function object that returns a yes/no answer, typically as a bool value. A function is pure in the mathematical sense if its result depends only on its arguments (note that this use of "pure" has nothing to do with pure virtual functions).

Don't allow predicates to hold or access state that affects the result of their operator(), including both member and global state. Prefer to make operator() a const member function for predicates (see Item 15).