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

Question

Hello, 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,

Answer 1

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

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

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.

Answer 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 :-)