Const vs define and use of const

This is a discussion on Const vs define and use of const within the C Programming forums, part of the General Programming Boards category; Ok a silly question. I am pretty sure that is preferable to use const to declare constants rather than define ...

  1. #1
    Registered User Phoenix_Rebirth's Avatar
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    Const vs define and use of const

    Ok a silly question. I am pretty sure that is preferable to use const to declare constants rather than define which just replaces the text on your code with the corresponding label but I was wondering why in a header file of mine where I declare my const it doesnt let me use them for arrays in some structs i made

    i.e

    Code:
    const int BODY_SIZE = 251;			
    const int BUFFER_SIZE = 256;		
    const int LOGIN_NL = 16;			
    
    const int ERROR = -1;
    const int SUCCESS = 0;
    const int SEND = 1;
    const int READ = 2;
    char buf[BUFFER_SIZE];				
    
    typedef struct mail{
            char sender[LOGIN_NL];
            char receiver[LOGIN_NL];
            char text[BODY_SIZE];
            struct mail* next;
    }mail;
    the compiler says
    error: variably modified β...β at file scope

    Any help

  2. #2
    C++まいる!Cをこわせ!
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    Don't put any variables in headers.
    Define them in a .cpp file and use extern in the header files.

  3. #3
    Registered User Phoenix_Rebirth's Avatar
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    What do you mean use extern in header files. I declare them in a cpp file and redeclare them in a header with the keyword extern in front of them

  4. #4
    C++まいる!Cをこわせ!
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    Quote Originally Posted by Phoenix_Rebirth View Post
    I declare them in a cpp file and redeclare them in a header with the keyword extern in front of them
    That's how you do it, although you don't assign a value to them in the header:
    Code:
    // cpp:
    const int my_int = 0;
    // h:
    extern const int my_int;
    But if you are doing this, it wasn't clear in your original post.

  5. #5
    Registered User Phoenix_Rebirth's Avatar
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    still the same, i dont know. Maybe I need some other command for compiling them. So far I had three files:
    client.c
    functions_Client.h
    objects.h

    so I also make another file objects.c
    to compile them what do I write
    Before I wrote gcc -o client.out client.c and that was it. So do I need another parameter now that I have another c file

  6. #6
    and the hat of int overfl Salem's Avatar
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    You can't use const like that in a C program to declare the length of an array. It must be a #define or a literal constant like say arr[10]
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  7. #7
    Algorithm Dissector iMalc's Avatar
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    ... or an enum.

    using a const int like that will only work in C++ that I know of, though maybe C99 as well I don't know.
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