See Figure 8.1
These objects may be allocated on the stack at will, and are meant for parameter passing only. They should never occur as a data member except as part of another temporary. The suffix _a to the reference short name, here meaning auto, is meant to remind the programmer of this.
In most cases, to provide transparency, the ctor arguments are one-to-one with the data members, and to allow convenient allocation in expressions, there are no non-const member functions beyond the constructor. In short, as the file and class name indicates, they're just wrappers.