RAD Studio (Common)
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A pointer is a variable that denotes a memory address. When a pointer holds the address of another variable, we say that it points to the location of that variable in memory or to the data stored there. In the case of an array or other structured type, a pointer holds the address of the first element in the structure. If that address is already taken, then the pointer holds the address to the first element.
Pointers are typed to indicate the kind of data stored at the addresses they hold. The general-purpose Pointer type can represent a pointer to any data, while more specialized pointer types reference only specific types of data. Pointers occupy four bytes of memory.
This topic contains information on the following:
To see how pointers work, look at the following example.
1 var 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 := P^; // dereference P; assign the result to Y 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 ^P) 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 ^ has two purposes, both of which are illustrated in our example. When it appears before a type identifier
^typeName
it denotes a type that represents pointers to variables of type typeName. When it appears after a pointer variable
pointer^
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 = ^Integer; var R: Single; I: Integer; P: Pointer; PI: PInteger; begin ... P := @R; PI := PInteger(P); I := PI^; end;
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 P1^.Data^.
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.
You can declare a pointer to any type, using the syntax
type pointerTypeName = ^type
When you define a record or other data type, it might be useful to also to define a pointer to that type. This makes it easy to manipulate instances of the type without copying large blocks of memory.
The fundamental types PAnsiChar and PWideChar represent pointers to AnsiChar and WideChar values, respectively. The generic PChar represents a pointer to a Char (that is, in its current implementation, to an AnsiChar). These character pointers are used to manipulate null-terminated strings. (See Working with null-terminated strings.)
The $T compiler directive controls the types of pointer values generated by the @ operator. This directive takes the form of:
{$T+} or {$T-}
In the {$T-} state, the result type of the @ operator is always an untyped pointer that is compatible with all other pointer types. When @ is applied to a variable reference in the {$T+} state, the type of the result is ^T, where T is compatible only with pointers to the type of the variable.
The System and SysUtils units declare many standard pointer types that are commonly used.
Selected pointer types declared in System and SysUtils
Pointer type |
Points to variables of type |
PAnsiString, PString |
AnsiString |
PByteArray |
TByteArray (declared in SysUtils). Used to typecast dynamically allocated memory for array access. |
PCurrency, PDouble, PExtended, PSingle |
Currency, Double, Extended, Single |
PInteger |
Integer |
POleVariant |
OleVariant |
PShortString |
ShortString. Useful when porting legacy code that uses the old PString type. |
PTextBuf |
TTextBuf (declared in SysUtils). TTextBuf is the internal buffer type in a TTextRec file record.) |
PVarRec |
TVarRec (declared in System) |
PVariant |
Variant |
PWideString |
WideString |
PWordArray |
TWordArray (declared in SysUtils). Used to typecast dynamically allocated memory for arrays of 2-byte values. |
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