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.

@DatHa -- This is the full code. It's extremely boilerplate. :) — Fine Man
– Fine Man, Commented Dec 2, 2016 at 21:35

Majenko · Accepted Answer · 2016-12-02 21:37:19Z

2

#define XYZ
...
class XYZ { };

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

answered Dec 2, 2016 at 21:37

Majenko

106k5 gold badges82 silver badges139 bronze badges

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.

Fine Man
– Fine Man

2016-12-02 21:40:47 +00:00
Commented Dec 2, 2016 at 21:40
Do you know what #define XYZ actually does?

Majenko
– Majenko

2016-12-02 21:41:12 +00:00
Commented Dec 2, 2016 at 21:41
I think it (along with #ifndef and #endif) is suppose to keep from duplicate code during linking?

Fine Man
– Fine Man

2016-12-02 21:42:20 +00:00
Commented Dec 2, 2016 at 21:42
That is what it is being used for. I asked if you know what it does?

Majenko
– Majenko

2016-12-02 21:42:46 +00:00
Commented Dec 2, 2016 at 21:42
Not particularly. :)

Fine Man
– Fine Man

2016-12-02 21:43:10 +00:00
Commented Dec 2, 2016 at 21:43

| Show 3 more comments

jfpoilpret · Accepted Answer · 2016-12-02 21:47:24Z

3

#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.

answered Dec 2, 2016 at 21:47

jfpoilpret

9,1627 gold badges38 silver badges54 bronze badges

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).

Fine Man
– Fine Man

2016-12-02 21:48:36 +00:00
Commented Dec 2, 2016 at 21:48

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