Where can & can't variables be declared in C?

This is a discussion on Where can & can't variables be declared in C? within the C Programming forums, part of the General Programming Boards category; Hi, This is probably a simple question for most of you, but I was just wondering about the rules regarding ...

  1. #1
    and the hat of sweating
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    Where can & can't variables be declared in C?

    Hi,
    This is probably a simple question for most of you, but I was just wondering about the rules regarding where variables (inside functions) can be declared?

    I always thought they could only be declared at the top of a function before any real code, but I've seen variables declared further down in some C code (like inside loops...).

    Also, do different versions of C (like C99...) support putting variable declarations in different spots?

    And one other thing... what about variable initialization rules? Can any variable be initialized when it's declared (assuming other declarations follow it), or just ints?

  2. #2
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    In "standard C" (rather than C++) you can declare variables at the beginning of a block in a function, so essentially anywhere between a { and the first line of "actual code".

    So:
    Code:
    int main()
    {
        // Here
        some_code();
        // Note here. 
        { 
            /// Here
            some_function();
            // Not here
           switch(a)
           {
              case 1: // Not here. 
                   break;
              case 2:
                   { // Here 
                      x = 9;
                      // not here. 
                   }
                   break;
               case 3:
                    if (x == 9) {
                         int y = 43;
                         z = 11;
                         if (b)  // Not here. 
                         else {
                           // Here 
                         }
                      }
                      break;
            }
            // Artificial block for a variable 
            {
                int somevar;
                .... 
            }
    }
    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
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    Quote Originally Posted by matsp View Post
    In "standard C" (rather than C++) you can declare variables at the beginning of a block in a function, so essentially anywhere between a { and the first line of "actual code".

    So:
    Code:
    int main()
    {
        // Here
        some_code();
        // Note here. 
        ...
    }
    --
    Mats
    I think in C99 one can do this.. it works for me.. long ago I tdidnt..
    I am using GCC 4.1 on Linux
    C's Motto: who cares what it means? I just compile it!!

  4. #4
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    Thanks. I had a feeling it was something like that.
    I'd rather not use C99 features just yet, since some widely used compiler$ still don't support it.

    Any thoughts about what/when variables can be initialized during declaration?

  5. #5
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    Quote Originally Posted by cpjust View Post
    Thanks. I had a feeling it was something like that.
    I'd rather not use C99 features just yet, since some widely used compiler$ still don't support it.

    Any thoughts about what/when variables can be initialized during declaration?
    All variabels should be able to be initalized whenever they are declared. You can't call functions to initialize global variables in "C".

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  6. #6
    Frequently Quite Prolix dwks's Avatar
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    Code:
           switch(a)
           {
              case 1: // Not here. 
                   break;
    Actually, you can declare a variable in that particular place (at least according to Dinkumware), because it's the first case statement in the switch. But we know what you mean -- in general, a case statement doesn't constitute the beginning of a block, so you normally can't declare anything there.

    Code:
                         if (b)  // Not here.
    Why not?
    Code:
    if(b) int x;
    Of course, that wouldn't be very useful, because you couldn't use x at all . . .

    To state this slightly more formally: variable declarations must be the first code inside a block. A block is enclosed with {}, and also occurs when you put only one statement after an if or else clause, or a loop.
    Code:
    if(1) int x;
    do
        int x;
    while(0);
    while(1) int x;
    Of course, that's rarely (if ever) useful.

    One point that should be clarified is that a switch statement's block begins at the {, not at each individual case label.

    Also note that any code other than a declaration will end the "beginning" of the block and make it impossible to declare a variable. These are all declarations, and thus can be intermingled with variables:
    Code:
    void function(void);
    typedef int number;
    struct whatever {};
    enum { MAX = BUFSIZ };
    int x = y;
    int x = yes();
    double f = pow(2, 8);
    These are not declarations, they are statements:
    Code:
    x = 0;
    if(x) {}
    x = y;
    x = yes();
    dwk

    Seek and ye shall find. quaere et invenies.

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