Messing with header files

This is a discussion on Messing with header files within the C++ Programming forums, part of the General Programming Boards category; I have two .cpp files in my project. (Dev-C++, WinXP) They both include "main.h" header. No matter if the header ...

  1. #1
    Reverse Engineer maxorator's Avatar
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    Messing with header files

    I have two .cpp files in my project. (Dev-C++, WinXP)
    They both include "main.h" header.
    No matter if the header contains:
    Code:
    char hello[] = "Hello";
    or
    Code:
    #pragma once
    char hello[] = "Hello";
    or
    Code:
    #ifndef MYHEADER
    #define MYHEADER
    char hello[] = "Hello";
    #endif
    I always get such errors:
    Code:
    multiple definition of `hello'
    first defined here
    And why are those quotes so freaky?
    "The Internet treats censorship as damage and routes around it." - John Gilmore

  2. #2
    and the hat of wrongness Salem's Avatar
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    Because you should have in the header file
    Code:
    extern char hello[];
    And in ONE .cpp file,
    Code:
    char hello[] = "hello";
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
    Reverse Engineer maxorator's Avatar
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    Is that meant for every data type, IE:
    header:
    Code:
    extern int baa;
    ONE .cpp file:
    Code:
    int baa;
    "The Internet treats censorship as damage and routes around it." - John Gilmore

  4. #4
    CSharpener vart's Avatar
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    Right - for every global variable that you want to use in several c-files
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  5. #5
    Reverse Engineer maxorator's Avatar
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    Then I will make one header.cpp and main.h.
    "The Internet treats censorship as damage and routes around it." - John Gilmore

  6. #6
    CSharpener vart's Avatar
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    You can create something like global.cpp with all global variables you want
    and global.h that contains extern for each such variable...

    I personally prefer to build a struct, that includes as a members all global variables I need
    So I will have only one global variable and only one extern...
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  7. #7
    and the hat of wrongness Salem's Avatar
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    If you have any global variables at all, they all should be in main.cpp (ideally static), and everything else gets access to them via parameters or return results.

    A wide scattering of globals used for all sorts of purposes is poor design.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
    CSharpener vart's Avatar
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    ideally static
    ideally they should be local variables inside main()
    But we don't live in the perfect world
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  9. #9
    Frequently Quite Prolix dwks's Avatar
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    For constants and such, you could also put a static global variable in the header file:
    Code:
    static const char *version = "program version 1.00";
    This generates a new static variable for each file that includes the header.
    dwk

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  10. #10
    CSharpener vart's Avatar
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    They (static) have not to be constant. I don't see a reason for using this, but you can put line

    Code:
    static int i =0;
    in the header and you will have in each c-file including this header its own static int i that can be changed inside this c-file.
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  11. #11
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    But if they aren't constant they will behave differently than global variables, since modifying one in a source file will not affect those used by other source files.

    BTW, use of static for this purpose is deprecated in favor of the unnamed namespace.

  12. #12
    Cat without Hat CornedBee's Avatar
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    But the unnamed namespace in headers is strongly discouraged by many groups (such as Boost) because it causes problems with precompiled headers on quite a few compilers.

    It's also not needed for constants.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  13. #13
    Reverse Engineer maxorator's Avatar
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    If you have any global variables at all, they all should be in main.cpp (ideally static), and everything else gets access to them via parameters or return results.
    Now that's an idea! Make global variables that can't be accessed as global variables. Scattering is the point of global variables.
    "The Internet treats censorship as damage and routes around it." - John Gilmore

  14. #14
    Cat without Hat CornedBee's Avatar
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    And the curse. Limit their use as much as you can.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  15. #15
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    You'll often find, especially with good use of encapsulation and OOP, that global variables are completely unnecessary; they often can be replaced with private member variables.
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