Adding an include guard breaks the build

Question

I added #ifndef..#define..#endif to a file of my project and the compiler fails. As soon as I remove it or put any other name in the define it compiles fine. What could be the problem?

Sounds like the file is already declared, but I do not know where. I'm fine just removing it, but I really want to know why this is happening.

error: expected class-name before ‘{’ token
error: ‘QDesignerFormEditorInterface’ has not been declared

And a couple of other errors.

I am actually using an example from Qt, "Custom Widget Plugin Example".

The difference is I am using my own class for the custom widget (.h, .cpp and .ui file).

It might have to do with the file that has 2 includes, though that is how the example did it.

Answer 1

Is this macro used as an include guard? If so, it sounds like you're duplicating a name used elsewhere. This is a common problem when people don't think about the scope an include guard must have—you should include much more information in it than just the file name.

Include guard goals:

• generate once, when creating a header
• never have to think about again
• chance of duplicating is less than your chance of winning the lottery

Bad include guard names (for file "config.h"):

• CONFIG_H
  • much too general
• _CONFIG_H, CONFIG__H, CONFIG_H__, __CONFIG_H__, etc.
  • all reserved, still much too general
• PROJECT_CONFIG_H
  • better, much less likely to duplicate in unrelated projects
  • but still no path information, easy to duplicate in large projects

Good include guard names (for file "config.h"):

• PATE_20091116_142045
  • that's <last name>_<date>_<time>
  • no project, path, filename information even needed
  • easy to type
    • if your editor has an insert-date feature, you can "type" it very fast
  • easy to generate
    • include a sequence number when generating, if you need to generate more than one per second
  • strong guarantee of being universally unique
• INCLUDE_GUARD_726F6B522BAA40A0B7F73C380AD37E6B
  • generated from an actual UUID
    • strong guarantee of being universally unique
  • if it turns up unexpectedly, "INCLUDE_GUARD" is a good hint about what it is, while serving to put it in a separate namespace (though by convention rather than recognized by the language)
  • prepend a project name, if desired (which is often required by project guidelines for macros)
  • easy to write your own sample program to generate

Answer 2

If you add an #ifndef for a constant that's already defined, it will always validate to true. You say "the file is declared", but files do not get declared. It is really the constant that you place after #ifndef that you should be checking. Do a simple search through the whole source tree and double-check in what order your current #define appears.

And of course: is your code correct? Try with a rubbish name as constant, and place #endif right after it: if it still errors, you have typos (paste your code, if so). See also this post.

PS: I see that David Thornley was typing similar advice in a comment... sorry if this duplicates info in this thread.