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The const modifier

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• The const keyword
  • const:

    The keyword const is used to specify that a variable cannot be modified: Example: int a ; // a is an int variable int const a ; // a is an int variable that cannot be modified (It actually says that: the expression a cannot be modified) A variable defined with the const keyword can only be given an initial value

    Note:

    This feature is a language construct built into the C compiler In other words: When the C compiler detects that the program uses an assignment statement with a const modified variable, the C compiler will report an error.

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  • Example:

    int main(int argc, char *argv[]) { int const a = 4; // Initial value printf (" a = %d\n", a); a = 1; // Illegal !!! printf (" a = %d\n", a); }

    Result:

    UNIX>> gcc const1.c const1.c: In function 'main': const1.c:12: error: assignment of read-only variable 'a'

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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 Check out the compile error !

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• Postscript: alternate syntax
  • Alternate syntax to define a inmutable variable:

    const DataType varName ;

    Example:

    const int a ;

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  • Note:

    This syntax is preferred: DataType const varName ;

    This will be explained later in this webpage: click here