Why use ifdef in header

Your reason for not including b. When the compiler is done with a. It does not matter that a. The purpose of a header guard is to prevent a. Without the guard, declarations would get compiled multiple times, causing errors about multiple declarations of existing symbols. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?

Collectives on Stack Overflow. Learn more. Asked 8 years, 10 months ago. Active 5 years, 2 months ago. Viewed 56k times. Here a more clear example b. If you have any idea as to how to go about this feel free to tell me some. Improve this question. Add a comment. Active Oldest Votes. Improve this answer. JBentley JBentley 5, 5 5 gold badges 32 32 silver badges 69 69 bronze badges. When b. Show 4 more comments. In your case: b. William Seiti Mizuta William Seiti Mizuta 7, 2 2 gold badges 28 28 silver badges 23 23 bronze badges.

CIsForCookies Punch "one definition rule" into your favorite search engine. QuantumFool 3 3 gold badges 9 9 silver badges 29 29 bronze badges. Roy Roy 1, 1 1 gold badge 13 13 silver badges 11 11 bronze badges. Identifiers starting with an underscore are reserved; you shouldn't define them yourself. I know this is an old comment, but actually the underscore restriction only applies to "external identifiers" - identifiers that could end up in the compiled object's symbol table, i.

It does not apply to macro names. Is Stu's comment true? I just read stackoverflow. In practice such symbols may be used to define macros in the standard libraries, which could collide with user-defined symbols of the same form I look forward to a response in ; greetings, from the age of the rat plague! Exception: user-defined literals. Add a comment. This prevent from the multiple inclusion of same header file multiple time.

They are called ifdef or include guards. Mohit Jain Mohit Jain 6 6 bronze badges. The Overflow Blog. Does ES6 make JavaScript frameworks obsolete? Podcast Do polyglots have an edge when it comes to mastering programming Featured on Meta. Now live: A fully responsive profile.

Linked 1. Thus, a program that defines a variable identifier more than once will cause a compile error:. This can happen when a header file includes another header file which is common. First, main. Then main. This copies contents of square. Duplicate definitions and a compile error. Each file, individually, is fine. However, because main. If geometry. The good news is that we can avoid the above problem via a mechanism called a header guard also called an include guard. Header guards are conditional compilation directives that take the following form:.

All of your header files should have header guards on them. For example, square. Even the standard library headers use header guards. If you were to take a look at the iostream header file from Visual Studio, you would see:. If only the filename is used for the include guard e. If that happens, any file that includes directly or indirectly both config.