C++ Primer, Fifth Edition

7.5.6. Literal Classes

In § 6.5.2 (p. 239) we noted that the parameters and return type of a constexpr function must be literal types. In addition to the arithmetic types, references, and pointers, certain classes are also literal types. Unlike other classes, classes that are literal types may have function members that are constexpr. Such members must meet all the requirements of a constexpr function. These member functions are implicitly const (§ 7.1.2, p. 258).

An aggregate class (§ 7.5.5, p. 298) whose data members are all of literal type is a literal class. A nonaggregate class, that meets the following restrictions, is also a literal class: ...