Concepts of Programming Languages

294 Chapter 6 Data Types

6.11.5 Pointers in C and C++

In C and C++, pointers can be used in the same ways as addresses are used in

assembly languages. This means they are extremely flexible but must be used

with great care. This design offers no solutions to the dangling pointer or lost

heap-dynamic variable problems. However, the fact that pointer arithmetic is

possible in C and C++ makes their pointers more interesting than those of the

other programming languages.

C and C++ pointers can point at any variable, regardless of where it is allo-

cated. In fact, they can point anywhere in memory, whether there is a variable

there or not, which is one of the dangers of such pointers.

In C and C++, the asterisk (*) denotes the dereferencing operation, and

the ampersand (&) denotes the operator for producing the address of a variable.

For example, consider the following code:

int *ptr;

int count, init;

...
ptr = &init;
count = *ptr;

The assignment to the variable ptr sets it to the address of init. The assign-

ment to count dereferences ptr to produce the value at init, which is then

assigned to count. So, the effect of the two assignment statements is to assign

the value of init to count. Notice that the declaration of a pointer specifies

its domain type.

Notice that the two assignment statements above are equivalent in their

effect on count to the single assignment

count = init;

Pointers can be assigned the address value of any variable of the correct

domain type, or they can be assigned the constant zero, which is used for nil.

Pointer arithmetic is also possible in some restricted forms. For example,

if ptr is a pointer variable that is declared to point at some variable of some

data type, then

ptr + index

is a legal expression. The semantics of such an expression is as follows.

Instead of simply adding the value of index to ptr, the value of index is

first scaled by the size of the memory cell (in memory units) to which ptr

is pointing (its base type). For example, if ptr points to a memory cell for

a type that is four memory units in size, then index is multiplied by 4, and

the result is added to ptr. The primary purpose of this sort of address arith-

metic is array manipulation. The following discussion is related to single-

dimensioned arrays only.