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Understanding C's syntax for variable definition

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• How to read a variable definition clause in modern programming languages
  • Students in Emory studied programming using the modern Java programming language which has a simple to understand syntax for variable definition

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  • Java's syntax to define a variable:

    DataType variableName ;

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  • Examples:

    int a ; a is an int int[] b ; b is an int[] (array of int)

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  • Note:

    In Java, you can also define an array (object) variable as follows: int a[] ; This syntax is to accommodate the older generation of C style array definition. The proper form of array definition in Java is: int[] a ;

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• Reading C's syntax for variable definitions
  • How to read a simple variable definition in C:

    int a ; // meaning: the expression a is of the type int

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  • How to read a reference variable definition in C:

    int *a ; should be read as: int *a ; // meaning: the expression *a is an int

    $64,000 question:

    How can the fact that "*a is an int variable" be equivalent to the fact that "a is an reference variable to an int variable"

    Explanation:

    If: *a is an int (and * is the de-reference operator !) then: a contains an address of an int variable !!! i.e.: a is a reference variable to an int variable

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• More common way to write a definition of reference variables
  • Note:

    Some times, you see programmers use the following "style" (that is not used by mature C programmers) to write the definition of a reference variable: int* a ; The "intention is to mimic the Java style of variable definition to reflect the fact that the variable a has the type int*. The conventional (and proper) way in C to write the definition of a reference variable: int *a ; because this notation reflects the underlying meaning of the definition as: The expression   *a   is of the type int

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• Some more advanced examples of variable definition syntax
  • Consider the following variable definition that defines 2 variables:

    int *a, b ;

    This variable definition defines:

    *a is an int ===> a is a reference variable to an int b is an int ===> b is an int

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  • Example 2:

    int *a, *b ;

    This variable definition defines:

    *a is an int ===> a is a reference variable to an int *b is an int ===> b is also a reference variable an int

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• How to read variable definitions with the const modifier
  • Example:

    int a ; read as: a is an int int const a ; read as: const a is an int <==> a is an int and you cannot update a

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• More advanced example: reading variable definitions with the const modifier
  • Question: what kind of variable is a:

    int const *a ;

    Answer:

    int const *a means: const *a is an int <==> *a is an int and you cannot update *a <==> a is a reference to an int and you cannot update *a

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  • In other words:

    The variable a points to an integer variable that you cannot update that integer variable.

    Example:

    int main(int argc, char *argv[]) { int a; const int *p; // You CANNOT use *p to update the int value ! a = 4; p = &a; *p = 100; // Illegal !!! }

    Result:

    UNIX>> gcc const1.c const1.c: In function 'main': const1.c:15: error: assignment of read-only location '*p'

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc const1.c To run:          ./a.out

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• A variation of the previous example....

  • Question: what kind of variable is a:

    int * const a ;

    Answer:

    int * const a means: * const a is an int <==> const a is a reference variable to an int <==> a is a reference variable to an int and you cannot update a

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  • In other words:

    int * const a: The variable a points to an integer variable that you cannot make a point so another variable (= cannot update a)

    Example:

    int main(int argc, char *argv[]) { int a = 4; int b = 7; int * const p = &a; // p points to a, and you cannot update p printf ("a = %d, b = %d\n", a, b); *p = 555; // You CAN update *p (it is not the same as p) printf ("a = %d, b = %d\n", a, b); p = &b; // Illegal - can't update p /* -------------------------- Contrast with this: -------------------------- */ int const * q = &a; // *q is a const q = &b; // You CAN update q *q = 555; // Illegal: You CANNOT update *q }

    Result:

    UNIX>> gcc const2.c const2.c: In function 'main': const2.c:line 20: error: assignment of read-only variable 'p' const2.c:line 31: error: assignment of read-only location '*q'

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc const2.c To run:          ./a.out

    Exercise:

    Remove the offending lines, and recompile....