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How to read C's const variable definition

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

    The const modifier: .... const X specifies the fact that the expression: X (the expression after const) cannot be updated

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  • 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 p:

    int const *p ;

    Answer:

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

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  • In other words:

    The variable p points to an integer variable that you cannot use that *p to update that integer variable. (A nice safety measure !!!)

    Example:

    int main(int argc, char *argv[]) { int a; int const *p; // You CANNOT update *p ! 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 q:

    int * const q ;

    Answer:

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

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  • In other words:

    int * const q: The variable q points to a fixed integer variable You cannot make q point so another variable !!!

    Example:

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

    Result:

    UNIX>> gcc const2.c const2.c: In function 'main': const2.c:line 20: error: assignment of read-only variable 'q' const2.c:line 31: 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 const2.c To run:          ./a.out

    Exercise:

    Remove the offending lines, and recompile....