c++ - 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