Why can't my .cpp file have a class of the same name?

Question

I made a new Arduino IDE project ABC, and am given ABC.ino. Then I create a XYZ.h and XYZ.cpp.

---

ABC.ino:

#include "XYZ.h"
void setup() {}
void loop() {}

XYZ.h:

#ifndef XYZ
#define XYZ
#include "Arduino.h"
class XYZ { };
#endif

XYZ.cpp:

#include "Arduino.h"
#include "XYZ.h"

---

When I try to compile this, I get some error:

In file included from sketch/XYZ.cpp:2:0:
XYZ:8: error: abstract declarator '<anonymous class>' used as declaration
 };
 ^
exit status 1
abstract declarator '<anonymous class>' used as declaration

---

However, when I rename my class to something other than the file name, like XYZa, the compilation succeeds.

---

Why can't I have the same name class as the file?

I've done C++ projects (independent of Arduino) via Xcode and nano, and this has never been a problem.

• Dat Ha: INCLUDE THE FULL CODE!!!
• Fine Man: @DatHa -- This is the full code. It's extremely boilerplate. :)

Answer 1

#define XYZ
...
class XYZ { };

After preprocessing, what do you think that ends up as?

• Fine Man: IDK. But, if you're suggesting that it's because it's an empty class, I think I've ruled it out. If you add void QRS() to .h and void XYZ::QRS() {} to .cpp I still get that error AND another one.
• Majenko: Do you know what #define XYZ actually does?
• Fine Man: I think it (along with #ifndef and #endif) is suppose to keep from duplicate code during linking?
• Majenko: That is what it is being used for. I asked if you know what it does?
• Fine Man: Not particularly. :)

Answer 2

#define XYZ
class XYZ { };

The first line tells the "C preprocessor" (called before C and C++ compilers) to replace XYZ with nothing everywhere it happens, hence after preprocessing, and before compiling, that code becomes:

#define XYZ
class { };

This is where C++ compiler complains. It has nothing to do with your file name.

Among C and C++ programmers, it is commonly admitted that header files should be protected by macros named after the file name including the extension:

#ifndef XYZ_H
#define XYZ_H

If you use this practice, you won't have this problem anymore.

• Fine Man: That's what is inconsistent with my Arduino program and my other ones (Xcode/nano). I forgot to append the _H_ (or _H as you prefer).