CHAPTER 10: Separate Compilation and Libraries

10.6 Summary and Conclusion

• The compilation units of a program form a program library. Library units can be declarations of packages, subprograms or generic units, or generic instantiations. The bodies of library units are separately compiled secondary units.

• Subunits of other compilation units can be defined by means of body stubs. These subunits are separately compiled.

• The visibility rules applicable to compilation units are the usual visibility rules, as complemented by with clauses. The order of compilation and recompilation is governed by these rules.

Finally, one of the motivations of separate compilation is the creation of software libraries. This is supported by Ada. By far the most useful library units should be packages and generic packages. Ada permits their use with the same degree of safety as for internal units.

It is expected that library packages will be used for the encapsulation of type definitions, for common constants and data, and for shared declarations. The fact that these library items are already compiled program units, rather than source texts, offers a degree of safety not found in languages that provide merely independent compilations.

Other compilation units will be used for the creation of user packages such as input-output packages, to be found in libraries. The ability to compile a package specification separately from the corresponding package body provides the possibility of separating the interface of a package from its implementation. Thus it supports information hiding and reliability to an extremely high degree.