Question
I have been seeing code like this usually in the start of header files:
#ifndef HEADERFILE_H
#define HEADERFILE_H
And at the end of the file is
#endif
What is the purpose of this?
Answer
Those are called #include guards.
Once the header is included, it checks if a unique value (in this case
HEADERFILE_H) is defined. Then if it's not defined, it defines it and
continues to the rest of the page.
When the code is included again, the first ifndef fails, resulting in a
blank file.
That prevents double declaration of any identifiers such as types, enums and static variables.
