Free Pascal supports the use of pointers. A variable of the pointer type contains an address in memory, where the data of another variable may be stored.
Pointer types
As can be seen from this diagram, pointers are typed, which means that they point to a particular kind of data. The type of this data must be known at compile time.
Dereferencing the pointer (denoted by adding ^ after the variable name) behaves then like a variable. This variable has the type declared in the pointer declaration, and the variable is stored in the address that is pointed to by the pointer variable.
Consider the following example:
In this example, BP is a pointer to a Buffer type; while B
is a variable of type Buffer. B takes 256 bytes memory,
and BP only takes 4 bytes of memory (enough to keep an adress in
memory).
Remark: Free Pascal treats pointers much the same way as C does. This means
that you can treat a pointer to some type as being an array of this type.
The pointer then points to the zeroeth element of this array. Thus the
following pointer declaration
Can be considered equivalent to the following array declaration:
The difference is that the former declaration allocates memory for the
pointer only (not for the array), and the second declaration allocates
memory for the entire array. If you use the former, you must allocate memory
yourself, using the Getmem function.
The reference P^ is then the same as p[0]. The following program
illustrates this maybe more clear:
Free Pascal supports pointer arithmetic as C does. This means that, if P is a
typed pointer, the instructions
Will increase, respectively descrease the address the pointer points to
with the size of the type P is a pointer to. For example
will increase P with 4.
You can also use normal arithmetic operators on pointers, that is, the
following are valid pointer arithmetic operations:
Here, the value that is added or substracted is not multiplied by the
size of the type the pointer points to.
