in C++ , what's so special about "_MOVE_H"?

Question

I have a C++ file like this

#ifndef _MOVE_H
#define _MOVE_H
class Move {
 int x, y;
public:
 Move(int initX = 0, int initY = 0) : x(initX), y(initY) {}
 int getX() { return x; }
 void setX(int newX) { x = newX; }
 int getY() { return y; }
 void setY(int newY) { y = newY; }
};
#endif

And to my amazement, all the code between #ifndef and #endif is simply ignored by the compiler (I swear that I am not defining _MOVE_H anywhere else), and I have all kinds of errors about missing definitions. I was thinking that I did something wrong, but when I try to use another key (like _MOVE_Ha, everything is back to normal. Does _MOVE_H mean something special in C++ ?

I'm running Ubuntu 10.04, GCC 4.4.3, if that matters.

Thanks,

• Naveen: may be the one of the library files you are including using that for its inclusion guard? did you check that? btw, I normally prefix it with the project name so as to avoid exactly this type of issue.
• Inverse: Another reason to use #pragma once instead of #define include guards...
• Nathan Ernst: @Inverse, #pragma once is a compiler extension and not supported by all compilers. Include guards are the only safe compiler independent means of preventing multiple inclusion.
• Inverse: @Nathan Ernst: I know, but if you're using any of the 3 major compilers, #pragma once saves the compiler from having to open and process the same guarded header over and over every compile.
• MSalters: There's no need for a compiler to open and process a normal header either. The idea of recognizing include guards was invented and implemented over a decade ago, IIRC.

Answer 1

just run grep _MOVE_H in /usr/include/c++ on your machine

for me :

c++/4.5.0/bits/move.h:#ifndef _MOVE_H

As a rule of thumb, don't use things (really anything) prefixed by _ or __. It's reserved for internal usage. Use SOMETHING_MOVE_H (usually name of the company, ...).

I guess it's a new header used to add the move semantic to c++0x.

• Tyler McHenry: To clarify the underscore rule, the following things are reserved: any global name prefixed by _, any name of any sort prefixed by __, and any name of any sort prefixed by _ and beginning with a capital letter. The OP runs afoul of the first and third rules.
• Dennis Zickefoose: Any name containing a __ at all is reserved. It need not be prefixed.

Answer 2

Anything beginning with an underscore then capital letter is reserved to the implementation. (i.e. _M). I think in general you want to stay away from leading underscores.

Answer 3

I believe gcc has an include file called move.h that includes the sentinel _MOVE_H. Presumably you have collided with this. Use a different identifier, preferably one that doesn't start with an underscore. I put a GUID in mine, but then I'm really obsessive :-)

Answer 4

It's a trick to prevent the same header file being included more than once. The actual value you #define doesn't matter - so long as it's only defined in that header file, the convention is NAME_HEADER_FILE_H in capitals

See also this discussion on #pragma once

• phunehehe: sorry, that's not really the problem. the problem is, my file isn't being included at all.