Overview of pointers

To see how pointers work, look at the following example.

2

X, Y: Integer;

//

X and Y are Integer variables

3

P: "Integer

//

P points to an Integer

4

begin

5

X := 17;

//

assign a value to X

6

P := @X;

//

assign the address of X to P

7

Y := PA;

//

dereference P; assign the result

8

end;

Line 2 declares X and y as variables of type Integer. Line 3 declares p as a pointer to an Integer value; this means that p can point to the location of X or y. Line 5 assigns a value to X, and line 6 assigns the address of X (denoted by @x) to p. Finally, line 7 retrieves the value at the location pointed to by p (denoted by ap) and assigns it to y. After this code executes, X and y have the same value, namely 17.

The @ operator, which we have used here to take the address of a variable, also operates on functions and procedures. For more information, see The @ operator and Procedural types in statements and expressions.

The symbol A has two purposes, both of which are illustrated in our example. When it appears before a type identifier

AtypeName it denotes a type that represents pointers to variables of type typeName. When it appears after a pointer variable pointerA

it dereferences the pointer; that is, it returns the value stored at the memory address held by the pointer.

Our example may seem like a roundabout way of copying the value of one variable to another -something that we could have accomplished with a simple assignment statement. But pointers are useful for several reasons. First, understanding pointers will help you to understand the Delphi language, since pointers often operate behind the scenes in code where they don't appear explicitly. Any data type that requires large, dynamically allocated blocks of memory uses pointers. Long-string variables, for instance, are implicitly pointers, as are class instance variables. Moreover, some advanced programming techniques require the use of pointers.

Finally, pointers are sometimes the only way to circumvent Delphi's strict data typing. By referencing a variable with an all-purpose Pointer, casting the Pointer to a more specific type, and then dereferencing it, you can treat the data stored by any variable as if it belonged to any type. For example, the following code assigns data stored in a real variable to an integer variable.

type

PInteger = AInteger; var

R: Single; I: Integer; P: Pointer; PI: PInteger; begin

Of course, reals and integers are stored in different formats. This assignment simply copies raw binary data from r to I, without converting it.

In addition to assigning the result of an @ operation, you can use several standard routines to give a value to a pointer. The New and GetMem procedures assign a memory address to an existing pointer, while the Addr and Ptr functions return a pointer to a specified address or variable.

Dereferenced pointers can be qualified and can function as qualifiers, as in the expression PlA.DataA.

The reserved word nil is a special constant that can be assigned to any pointer. When nil is assigned to a pointer, the pointer doesn't reference anything.

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