int *ptr;
declares a pointer named ptrthat can point to a type intvariable. You then use the
address-of operator (&) to make the pointer point to a specific variable of the correspond-
ing type. Assuming that myVarhas been declared as a type intvariable, the statement
ptr = &myVar;
assigns the address of myVartoptrand makes ptrpoint to myVar. Again, using the indi-
rection operator, you can access the pointed-to variable by using its pointer. Both of the
following statements assign the value 12 tomyVar:
myVar = 12;
*ptr = 12;
Because a pointer is itself a numeric variable, it is stored in your computer’s memory at a
particular address. Therefore, you can create a pointer to a pointer, a variable whose
value is the address of a pointer. Here’s how:
int myVar = 12; /* myVar is a type int variable. */
int *ptr = &myVar; /* ptr is a pointer to myVar. */
int **ptr_to_ptr = &ptr; /* ptr_to_ptr is a pointer to a */
/* pointer to type int. */
Note the use of a double indirection operator (**) when declaring a pointer to a pointer.
You also use the double indirection operator when accessing the pointed-to variable with
a pointer to a pointer. Thus, the statement
**ptr_to_ptr = 12;
assigns the value 12 to the variable myVar, and the statement
printf(“%d”, **ptr_to_ptr);
displays the value of myVaron-screen. If you mistakenly use a single indirection opera-
tor, you get errors. The statement
*ptr_to_ptr = 12;
assigns the value 12 toptr, which results in ptr’s pointing to whatever happens to be
stored at address 12. This clearly is a mistake.
Declaring and using a pointer to a pointer is called multiple indirection. Figure 15.1
shows the relationship between a variable, a pointer, and a pointer to a pointer. There’s
really no limit to the level of multiple indirection possible—you can have a pointer to a
pointer to a pointer ad infinitum,but there’s rarely any advantage to going beyond two
levels; the complexities involved are an invitation to mistakes.388 Day 1525 448201x-CH15 8/13/02 11:13 AM Page 388