Effective C++: 55 Specific Ways to Improve Your Programs and Designs, Third Edition (Gal Zentner's Library) by Scott Meyers

Author:Scott Meyers
Language: eng
Format: epub
Publisher: Addison-Wesley Professional
Published: 2011-03-19T16:00:00+00:00

class Date; // class declaration

Date today(); // fine — no definition

void clearAppointments(Date d); // of Date is needed

Of course, pass-by-value is generally a bad idea (see Item 20), but if you find yourself using it for some reason, there's still no justification for introducing unnecessary compilation dependencies.

The ability to declare today and clearAppointments without defining Date may surprise you, but it's not as curious as it seems. If anybody calls those functions, Date's definition must have been seen prior to the call. Why bother to declare functions that nobody calls, you wonder? Simple. It's not that nobody calls them, it's that not everybody calls them. If you have a library containing dozens of function declarations, it's unlikely that every client calls every function. By moving the onus of providing class definitions from your header file of function declarations to clients' files containing function calls, you eliminate artificial client dependencies on type definitions they don't really need.

• Provide separate header files for declarations and definitions. In order to facilitate adherence to the above guidelines, header files need to come in pairs: one for declarations, the other for definitions. These files must be kept consistent, of course. If a declaration is changed in one place, it must be changed in both. As a result, library clients should always #include a declaration file instead of forward-declaring something themselves, and library authors should provide both header files. For example, the Date client wishing to declare today and clearAppointments shouldn't manually forward-declare Date as shown above. Rather, it should #include the appropriate header of declarations:

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