variables across files

This is a discussion on variables across files within the C++ Programming forums, part of the General Programming Boards category; Hey guys, If I declare a global variable in, say, "main.cpp", then try to access it in, say, "file2.cpp", it ...

  1. #1
    Carnivore ('-'v) Hunter2's Avatar
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    variables across files

    Hey guys,

    If I declare a global variable in, say, "main.cpp", then try to access it in, say, "file2.cpp", it won't work (thats right, right?). Earlier on, I read in a book that you can declare all of your global variables in a header, then include it in another file, then declare it using "extern *variablename*" in the second file... then, it said something like you can include the header in yet another file and use extern again, and then both files will have access to the globals. I tried it, but then it gave me an error:

    Missing ';' before identifier 'Ship' //this was in globals.h

    That usually means that 'Ship''s header hasn't been included yet... but it WAS. So then I tried not including the header with the globals (as in, I included it in 'main.cpp' but not in the second file), and then deleted all the stuff in the second file that depended on these global variables, and then it worked. Does anybody know why? (Or, for that matter, how I can share the global variables between files?)
    Just Google It. √

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  2. #2
    and the hat of wrongness Salem's Avatar
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    This goes in a header file, which is included in all the source files which access foo
    extern int foo;

    This goes in ONE source file to declare the variable
    int foo;

  3. #3
    Carnivore ('-'v) Hunter2's Avatar
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    so...
    Code:
    //global.h
    
    int foo;
    Code:
    //main.cpp
    
    #include "global.h"
    
    extern int foo;
    Code:
    //file2.cpp
    
    #include "global.h"
    
    extern int foo;
    right?... I tried that, but then it gave me an error in global.h...
    Just Google It. √

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  4. #4
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    show more of your global.h file.. what you showed is perfectly fine.. what is the current error + show the lines around the line it reports the error on.

  5. #5
    Carnivore ('-'v) Hunter2's Avatar
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    All right... this is globals.h
    Code:
    #ifndef _globals_h_
    #define _globals_h_
    
    bool done;
    GFX gfx;
    Ship ship; // <--- error here: "Missing ';' before identifier 'Ship'
    std::vector<Alien> aliens;
    
    #endif
    and this is the first part of main.cpp
    Code:
    #include <windows.h>
    #include "resource.h"
    
    #include "GFX.h"
    #include "Ship.h"
    #include "Bullet.h"
    #include "Alien.h"
    
    #include "main.h"
    #include "StaticDefs.h"
    #include "globals.h"
    
    extern bool done;
    extern GFX gfx;
    extern Ship ship;
    extern std::vector<Alien> aliens;
    and this is part of Rocket.cpp
    Code:
    #include "Rocket.h"
    #include <math.h>
    #include "CoordGeom.h"
    #include "Alien.h"
    #include <vector>
    #include "globals.h"
    
    extern std::vector<Alien> aliens;
    Just Google It. √

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  6. #6
    . Driveway's Avatar
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    This is probably because you did not link the class ship correctly

  7. #7
    Carnivore ('-'v) Hunter2's Avatar
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    *I tried including Ship.h before globals.h in Rocket.cpp, but after it got past that one error, it gave me 4 more linker errors:

    Code:
    "bool  done" (?done@@3_NA) already defined in main.obj
    "class Ship  ship" (?ship@@3VShip@@A) already defined in main.obj
    //etc.
    Last edited by Hunter2; 07-20-2002 at 07:43 PM.
    Just Google It. √

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  8. #8
    and the hat of wrongness Salem's Avatar
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    > right?... I tried that, but then it gave me an error in global.h...

    extern goes in front of the declarations in the .h files, not the .cpp files

    #ifndef _globals_h_
    #define _globals_h_

    // add extern here
    extern bool done;
    extern GFX gfx;
    extern Ship ship;
    extern std::vector<Alien> aliens;

    #endif

    and this is the first part of main.cpp

    #include <windows.h>
    #include "resource.h"

    #include "GFX.h"
    #include "Ship.h"
    #include "Bullet.h"
    #include "Alien.h"

    #include "main.h"
    #include "StaticDefs.h"
    #include "globals.h"

    // remove extern from these
    bool done;
    GFX gfx;
    Ship ship;
    std::vector<Alien> aliens;

    and this is part of Rocket.cpp

    #include "Rocket.h"
    #include <math.h>
    #include "CoordGeom.h"
    #include "Alien.h"
    #include <vector>
    #include "globals.h"

    // no need for this, its in globals.h
    // extern std::vector<Alien> aliens;

  9. #9
    Carnivore ('-'v) Hunter2's Avatar
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    Ok, thanks! I'll try that later and if I have any problems, I'll post another message
    Just Google It. √

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  10. #10
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    You could also do it this slick way...
    Code:
    #ifndef GLOBALS_H
    #define GLOBALS_H
    
    #ifdef MAIN_CPP
    #define GLOBAL 
    #else
    #define GLOBAL extern
    #endif
    
    GLOBAL UINT  g_Money;
    GLOBAL BOOL g_Loop;
    
    #endif /* GLOBALS_H */
    That way you don't have to type it twice. Maybe I'm just lazy. Right before you include this header in your main just add #define MAIN_CPP
    "...the results are undefined, and we all know what "undefined" means: it means it works during development, it works during testing, and it blows up in your most important customers' faces." --Scott Meyers

  11. #11
    Carnivore ('-'v) Hunter2's Avatar
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    Uh... what? Sorry, that #define stuff sorta confuses me... I'll have it sorted out in a couple of days
    Just Google It. √

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  12. #12
    Carnivore ('-'v) Hunter2's Avatar
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    Thanks Salem, it worked great! And thanks too, mr.wizard, although I couldn't figure out how yours was supposed to work
    Just Google It. √

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  13. #13
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    Originally posted by Hunter2
    Thanks Salem, it worked great! And thanks too, mr.wizard, although I couldn't figure out how yours was supposed to work
    Awww Well if you're curious about that implementation in your specific case, send me a message and I will help you. If you don't care, that's cool too

  14. #14
    Carnivore ('-'v) Hunter2's Avatar
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    Well... since Salem's worked fine, I'd have to say the latter :P
    Just Google It. √

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