7

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,

phunehehe
  • 8,618
  • 7
  • 49
  • 79
  • 1
    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. – Naveen Jul 27 '10 at 15:28
  • Another reason to use `#pragma once` instead of `#define` include guards... – Inverse Jul 27 '10 at 16:02
  • 4
    @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. – Nathan Ernst Jul 27 '10 at 16:31
  • @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. – Inverse Jul 28 '10 at 03:54
  • 1
    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. – MSalters Jul 28 '10 at 09:21

4 Answers4

16

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.

Scharron
  • 17,233
  • 6
  • 44
  • 63
  • 12
    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. – Tyler McHenry Jul 27 '10 at 15:32
  • 6
    Any name containing a `__` at all is reserved. It need not be prefixed. – Dennis Zickefoose Jul 27 '10 at 17:54
6

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.

James
  • 2,454
  • 1
  • 22
  • 22
4

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 :-)

Bob Moore
  • 6,788
  • 3
  • 29
  • 42
0

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

Community
  • 1
  • 1
Martin Beckett
  • 94,801
  • 28
  • 188
  • 263