Why are #ifndef and #define used in C++ header files?

c++ c c-preprocessor
Asad Khan picture Asad Khan · Oct 31, 2009 · Viewed 448.8k times · Source

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

LiraNuna picture LiraNuna · Oct 31, 2009

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.

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