External Variables

This is a discussion on External Variables within the C++ Programming forums, part of the General Programming Boards category; I've been having a bit of trouble with external variables, and I was wondering whether anyone had an idea why ...

  1. #1
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    External Variables

    I've been having a bit of trouble with external variables, and I was wondering whether anyone had an idea why this doesn't compile. (This is just example code, though it has the same errors).
    Code:
    //GLOBALS.H////////////////////////
    #pragma once
    
    extern const int MAPH;
    extern const int MAPW;
    extern const int CELLSIZE;
    ////////////////////////////////////////
    //GLOBALS.CPP////////////////////
    #include "globals.h"
    
    const int MAPH = 40;
    const int MAPW = 60;
    const int CELLSIZE = 10;
    ///////////////////////////////////////
    //FOO.H/////////////////////////////
    #pragma once
    #include "globals.h"
    
    void foo(int arr[][MAPH]);
    ///////////////////////////////////////
    //FOO.CPP/////////////////////////
    #include "globals.h"
    #include "foo.h"
    
    void foo(int arr[][MAPH])
    {
    //do whatever
    }
    //////////////////////////////////////
    //MAIN.CPP////////////////////////
    #include "globals.h"
    #include "foo.h"
    
    int main()
    {
            int arr[MAPW][MAPH];
            foo(arr);
            return 0;
    }
    //////////////////////////////////////
    As usuall, any help at all appreciated.

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by Raptoricus
    I've been having a bit of trouble with external variables, and I was wondering whether anyone had an idea why this doesn't compile. (This is just example code, though it has the same errors).
    You might want to state what those errors are.

    The problem is probably because the value of MAPH is not known when compiling foo.cpp, and the values of MAPH and MAPW are not known when compiling main.cpp, so it is as if you were trying to use variable length arrays. They are only known when linking the object file generated by compiling globals.cpp, but by then it is too late.
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    Quote Originally Posted by laserlight View Post
    You might want to state what those errors are.
    Not a problem:
    variable-size type declared outside of any function <- this appears in a functions header file (the function is practically the same as foo, with a multi dimensional array, using a global variable for bounds)

    What you're saying makes sense, but I'm not too sure how to fix the problem, any advice?
    Last edited by Raptoricus; 07-19-2009 at 01:38 AM. Reason: Tested

  4. #4
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by Raptoricus
    What you're saying makes sense, so would moving the includes around fix the problem?
    Not likely, and generally requiring a specific order of inclusion leads to fragile code and so is a Bad Thing.

    One way out, with minimal change to your current code, would be to use an enumeration in globals.h and get rid of globals.cpp:
    Code:
    enum {MAPH = 40, MAPW = 60, CELLSIZE = 10};
    Incidentally, note that fully capitalised names are typically reserved for macro names.
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    Okay thanks, I'll try that .

    Quote Originally Posted by laserlight View Post
    Incidentally, note that fully capitalised names are typically reserved for macro names.
    Oh my bad, my tutor says to use something like"const int kVARIABLE", to be honest though, I've just been trying to get the code to work, and not worrying so much about style. Still bad style leads to bad code I suppose, so thanks for pointing it out.

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    It worked! Thanks very much for your help.

  7. #7
    Cat without Hat CornedBee's Avatar
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    All-caps is very common for constants, simply because constants in C are macros and thus capitalized. The convention simply carried over to C++.
    All the buzzt!
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