Is int i; is a declaration or definition?

This is a discussion on Is int i; is a declaration or definition? within the C Programming forums, part of the General Programming Boards category; int i; printf("%d",i); If you execute above code, you will get some junk value as output. It means memory was ...

  1. #1
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    Is int i; is a declaration or definition?

    int i;
    printf("%d",i);

    If you execute above code, you will get some junk value as output. It means memory was allocated to i. Hence it is definition. But, many book refers its declaration as the value is explicitly not assigned to i. So, is this declaration or definition? Or depending upon compiler implementation it is declaration if memory is not allocated.

  2. #2
    Massively Single Player AverageSoftware's Avatar
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    That is a definition, and every definition is also a declaration.

    In order to declare but not define a variable, you do this:

    Code:
    extern int x;

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    extern int i;
    i=50;

    it wont work You have to initialise the variable in differnt scope then

  4. #4
    Woof, woof! zacs7's Avatar
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    No it's a declaration, your declaring 'i' as an integer. But your not defining it a value.

    A declaration and a definition would be:
    Code:
    int i = 10;
    i has been declared as an integer, and defined as 10.

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    Decralation and Defination are terms, that we use, not the compiler.
    S_ccess is waiting for u. Go Ahead, put u there.

  6. #6
    Massively Single Player AverageSoftware's Avatar
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    Quote Originally Posted by zacs7 View Post
    No it's a declaration, your declaring 'i' as an integer. But your not defining it a value.

    A declaration and a definition would be:
    Code:
    int i = 10;
    i has been declared as an integer, and defined as 10.
    You're confusing definition and initialization.

    All a declaration does is introduce a name into the compiler's symbol table. It doesn't generate any code when compiled.

    Code:
    extern int i;
    This informs the compiler that i is an int. It allocates no space for i, and it now falls upon the linker to actually locate the storage space that is i. It usually exists in a separate file, or more accurately, a separate translation unit.

    Code:
    void funk(int x);
    This is a function declaration. Once again, this simply informs the compiler that funk is void function that takes an int. The compiler does not know where the instructions that make up funk() actually are, but it knows enough to check the syntax.

    Code:
    class Thing;
    This is a class declaration. The compiler now knows the Thing is a class, which allows you to define Thing pointers and references, despite the compiler having never seen the definition of Thing.

    Code:
    extern int x = 0;
    This is perfectly legal and will compile, but it will not link(important distinction) unless x is defined somewhere. Most compilers have an option to compile, but not link a file. Experiment with this and it may become more clear.

  7. #7
    Code Goddess Prelude's Avatar
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    >So, is this declaration or definition?
    It's both. Your book (assuming it's a good one) probably meant the way that i was declared and not the point at which it was defined. There's a subtle difference there if you look at silly--but legal--code like this:
    Code:
    #include <stdio.h>
    
    int i;
    int i;
    int i;
    int i;
    int i;
    int i;
    
    int main ( void )
    {
      i = 10;
    
      printf ( "%d\n", i );
    
      return 0;
    }
    My best code is written with the delete key.

  8. #8
    Woof, woof! zacs7's Avatar
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    Quote Originally Posted by AverageSoftware
    You're confusing definition and initialization.
    No, think of it mathematically. If you 'initialize' a variable with a value your still defining it regardless if it's at 'declaration' time or not.
    Code:
    int i;
    i = 10;
    * i is declared as an integer
    * i is defined as 10

    IS the same as
    Code:
    int i = 10;
    * i is declared as an integer
    * i is defined as 10

    yes your initializing it with a value, but your still technically 'defining' i...

  9. #9
    C++ Witch laserlight's Avatar
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    No, think of it mathematically. If you 'initialize' a variable with a value your still defining it regardless if it's at 'declaration' time or not.
    AverageSoftware's point is that to initialise is to define, but the lack of an initialisation does not mean the lack of a definition. You stated: "No it's a declaration, your declaring 'i' as an integer. But your not defining it a value."

    In other words, you claim that:
    int i;
    is not a definition, which is false. It may be true in your "mathematical" terms, but then what's the equivalent of printf("&#37;d",i) in mathematics? This is C programming, not mathematics. Here is what C99 (section 6.7) has to say about the topic:

    A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:
    — for an object, causes storage to be reserved for that object;
    — for a function, includes the function body;
    — for an enumeration constant or typedef name, is the (only) declaration of the identifier.
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  10. #10
    Woof, woof! zacs7's Avatar
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    Although, you don't assign 'i' a value (but it will have one from the last value in at that address) it is still your still technically 'defining' 'i' by declaring it...?

    While it has a value, I wouldn't say you've defined it... I don't mean to be a pain, but I'm trying to understand as to what you mean

    But looking back on the OPs question I see where you're coming from
    Last edited by zacs7; 05-30-2007 at 01:30 AM.

  11. #11
    Massively Single Player AverageSoftware's Avatar
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    Quote Originally Posted by zacs7 View Post
    No, think of it mathematically. If you 'initialize' a variable with a value your still defining it regardless if it's at 'declaration' time or not.
    I'm thinking of it in terms of the C and C++ standards, where "declare" and "define" (and "initialize") have very specific meanings. It is likely those meanings that the original poster's book was referring to.

  12. #12
    Woof, woof! zacs7's Avatar
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    Quote Originally Posted by AverageSoftware View Post
    I'm thinking of it in terms of the C and C++ standards, where "declare" and "define" (and "initialize") have very specific meanings. It is likely those meanings that the original poster's book was referring to.
    Yeah, Sorry if I was an ass, I was thinking in mathematical terms

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