Am I missing something with const?

This is a discussion on Am I missing something with const? within the C++ Programming forums, part of the General Programming Boards category; Is there a case where const is supposed to act like static? I've defined a global constant in one file ...

  1. #1
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    Am I missing something with const?

    Is there a case where const is supposed to act like static? I've defined a global constant in one file and try to extern it in another and my linker fails. If I remove the const keyword, my program links.

    Here is an example of the code that fails to link

    File 1
    Code:
    #include <stdio.h>
    
    int main(void)
    {
      extern const int my_const;
      
      printf("hello %d\n", my_const);
      return 0;
    }
    File 2
    Code:
    const int my_const = 7;
    Simply removing the const keyword from both files allows me to link.

  2. #2
    a_capitalist_story
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    Compiles, links, and runs fine for me in gcc. What are you using to build?

    Code:
    gcc -Wall -pedantic -o file1 file1.c file2.c
    Code:
    $ ./file1
    hello 7

  3. #3
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    Quote Originally Posted by homer_3 View Post
    Is there a case where const is supposed to act like static?
    "static" has different meanings.

    Anything that you declare globally as const is static by default (has internal linkage, which means that other translation units can't see it). Put your "my_const" in a header file and do not define it in cpp file.
    Code:
    #ifndef HEADER_HPP
    #define HEADER_HPP
    const int my_const = 7; // no "static" or "extern" here
    #endif
    Anything that you declare locally isn't static. Static here has a different meaning. It means that you will have one variable shared between all calls to the same function. So if you want my_const to be visible only in main(), put:
    Code:
    int main(void)
    {
        static const int my_const = 7; // no "extern" here
    }
    Quote Originally Posted by homer_3 View Post
    Simply removing the const keyword from both files allows me to link.
    Removing "const" made "my_const" to have external linkage in the second file.
    Last edited by kmdv; 04-03-2013 at 01:44 AM.
    I never put signature, but I decided to make an exception.

  4. #4
    Registered User whiteflags's Avatar
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    Well C and C++ have some different rules regarding this: Linxutopia - Thinking in C++ - 3: The C in C++ - Linkage. And in C++ there is a whole other feature, extern "C" that you are not using as well, just to make a note of it.

    I created an example for you. Typically external linkage works like this, especially if you want the same global constant to be referenced around the program. The only way the linker will be able to resolve all of the references to the same variable is if there is a note about external linkage when the variable is actually defined. Where it is otherwise used, an extern declaration is fine.

    foo.h contains:
    Code:
    #ifndef FOO_H_INCLUDED
    #define FOO_H_INCLUDED
    
    extern const int my_const;
    
    void foo();
    
    #endif
    file1.cpp contains:
    Code:
    #include <stdio.h>
    #include "foo.h"
    
    const int my_const = 7;
    
    int main()
    {
        printf("hello %d\n", my_const);
        foo();
        return 0;
    }
    file2.cpp contains:
    Code:
    #include <stdio.h>
    #include "foo.h"
    
    void foo()
    {
        printf("from foo(), %d\n", my_const);
    }
    And with this you can clearly tell we are referencing the same constant, defined in file1.cpp. It could also be defined in file2.cpp if that makes more logical sense, there is no technical difference.

    static, as it relates to the topic, refers to static linkage.
    Last edited by whiteflags; 04-03-2013 at 02:17 AM.

  5. #5
    a_capitalist_story
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    Damn, I was totally thrown by the #include <stdio.h> and printf and compiled as C :/

  6. #6
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    Thanks for the clarification. I Googled around for something like that Linxutopia link, but couldn't find anything.

    It seems quite confusing to me that const means static const, when used at global scope, but I guess that's how it is. I also didn't realize defining a variable with extern was legal in C++. I always thought it was only for declarations.

    @Whiteflags, that's how I would normally do it, since it's just easier to use in general. But I thought what I had should work, obviously I was wrong.

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