C++ Primer Plus, Fourth Edition

Thirteen Types and a Constant

The basic_string template defines several types that are used later in defining the methods:

typedef traits                                traits_type;
typedef typename traits::char_type            value_type;
typedef Allocator                             allocator_type;
typedef typename Allocator::size_type         size_type;
typedef typename Allocator::difference_type   difference_type;
typedef typename Allocator::reference         reference;
typedef typename Allocator::const_reference   const_reference;
typedef typename Allocator::pointer           pointer;
typedef typename Allocator::const_pointer     const_pointer;

Note that traits is a template parameter that will correspond to some specific type, such as char_traits<char>; traits_type will become a typedef for that specific type. The notation:

 typedef typename ...