C++ Coding Standards: 101 Rules, Guidelines, and Best Practices

24. Always write internal #include guards. Never write external #include guards

Summary

Wear head(er) protection: Prevent unintended multiple inclusions by using #include guards with unique names for all of your header files.

Discussion

Each header file should be guarded by an internal #include guard to avoid redefinitions in case it is included multiple times. For example, a header file foo.h should follow the general form:

Observe the following rules when defining include guards:

• Use a unique guard name: Make sure it is unique at least within your application. We used a popular convention above; the guard name can include the application ...