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.

友情链接