What is a const pointer?


The access modifier keyword const is a promise the programmer makes to the compiler that the value of a variable will not be changed after it is initialized. The compiler will enforce that promise as best it can by not enabling the programmer to write code which modifies a variable that has been declared const.

A "const pointer," or more correctly, a "pointer to const," is a pointer which points to data that is const(constant, or unchanging). A pointer to const is declared by putting the word const at the beginning of the pointer declaration. This declares a pointer which points to data that can't be modified. The pointer itself can be modified. The following example illustrates some legal and illegal uses of a const pointer:

const char  *str = "hello";
char  c = *str    /* legal */
str++;            /* legal */
*str = 'a';       /* illegal */
str[1] = 'b';     /* illegal */

The first two statements here are legal because they do not modify the data that str points to. The next two statements are illegal because they modify the data pointed to by str.

Pointers to const are most often used in declaring function parameters. For instance, a function that counted the number of characters in a string would not need to change the contents of the string, and it might be written this way:

my_strlen(const char *str)
        int count = 0;
        while (*str++)
        return count;

Note that non-const pointers are implicitly converted to const pointers when needed, but const pointers are not converted to non-const pointers. This means that my_strlen() could be called with either a const or a non-const character pointer.


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