C++ Primer Plus

Functor Concepts

Just as the STL defines concepts for containers and iterators, it defines functor concepts:

• A generator is a functor that can be called with no arguments.

• A unary function is a functor that can be called with one argument.

• A binary function is a functor that can be called with two arguments.

For example, the functor supplied to for_each() should be a unary function because it is applied to one container element at a time.

Of course, these concepts come with refinements:

• A unary function that returns a bool value is a predicate.

• A binary function that returns a bool value is a binary predicate.

Several STL functions require predicate or binary predicate arguments. For example, Listing 16.9 uses a version of sort() that ...