C++ #ifndef for include files, why is all caps used for the header file?

Question

I wonder why the name after the #ifndef directive is always all caps and don\'t seem to match the name of the actual header file? What are the rules surrounding

Answer 1

It's completely subjective and there are no enforced rules other than those normally associated with the character set for naming pre-processor macros. It's conventional for macros to be defined in upper case. This tends to help them stand out in source code. A convention I tend to stick to is the strict capitalised version of the filename with the period replaced by an underscore and leading and trailing underscores. So, for a file called DataTableNameMangler.hpp the include guard would look like:

#ifndef _DATATABLENAMEMANGLER_HPP_
#define _DATATABLENAMEMANGLER_HPP_

...

#endif // _DATATABLENAMEMANGLER_HPP_

There's no great reason for this though I strongly recommend for consistency that the name matches the filename exactly. I normally use a little class creator script to generate my initial classes. The following Bash snippet gives an idea:

#!/bin/bash
INC_GUARD_NAME="_${1^^*}_HPP_"
echo "#ifndef $INC_GUARD_NAME"
echo "#ifndef $INC_GUARD_NAME"
echo
echo "class $1 {};"
echo
echo "#endif // $INC_GUARD_NAME"

Thus:

$ ./makeclass.bash DataTableNameMangler
#ifndef _DATATABLENAMEMANGLER_HPP_
#ifndef _DATATABLENAMEMANGLER_HPP_

class DataTableNameMangler {};

#endif // _DATATABLENAMEMANGLER_HPP_

This is naturally just a very basic example. Importantly, remember to put the comment before the guard name on the last line. #endif takes no parameters so the macro will be passed on to the C++ compiler which will complain about it if it's not commented.