The three class types are structure, class, and union. They are declared using the struct, class, and union keywords (see class-key syntax). Table 8.1 shows the differences among the three class types.
Table 8.1 Access Control and Constraints of Structures, Classes, and Unions
| Structures | Classes | Unions |
| class-key is struct | class-key is class | class-key is union |
| Default access is public | Default access is private | Default access is public |
| No usage constraints | No usage constraints | Use only one member at a time |