Pointer Arithmetic

• Source: https://www.freebasic.net/wiki/wikka.php?wakka=ProPgPtrArithmetic
• Last revised: 2019-07-04

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Manipulating address values mathematically.

Overview

Adding and subtracting from pointers

Incrementing and decrementing pointers

Distance between two pointers

Overview

It is often useful to iterate through memory, from one address to another. Pointers are used to accomplish this. While the type of a pointer determines the type of variable or object retrieved when the pointer is dereferenced (using Operator * (Value of)), it also determines the distance, in bytes, its particular type takes up in memory. For example, a Short takes up two (2) bytes in memory, while a Single needs four (4) bytes.

Adding and subtracting from pointers

Pointers can be added to and subtracted from just like a numeric type. The result of this addition or subtraction is an address, and the type of pointer determines the distance from the original pointer.

For example, the following,

start GeSHi

Dim p As Integer Ptr = New Integer[2]

*p = 1
*(p + 1) = 2

end GeSHi

will assign the values "1" and "2" to each integer in the array pointer to by p. Since p is an Integer Pointer, the expression "*(p + 1)" is saying to dereference an Integer four/eight (4/8 on 32/64bit systems) bytes from p; the "1" indicates a distance of "1 * the size of an Integer", or four/eight (4/8 on 32/64bit systems) bytes.

Subtraction follows the exact same principle. Remember, a - b = a + -b.

Incrementing and decrementing pointers

Sometimes it is more convenient to modify the pointer itself, in which case the combination addition and subtraction operators will work just like above. For example, the following,

start GeSHi

Dim array(5) As Short = { 32, 43, 66, 348, 112, 0 }
Dim p As Short Ptr = @array(0)

While (*p <> 0)
    If (*p = 66) Then Print "found 66"
    p += 1
Wend

end GeSHi

iterates through an array until it finds an element with the value of "0". If it finds an element with the value "66" it displays a nice message.

Distance between two pointers

The distance between two pointers is retrieved with Operator - (Subtract), and is measured in values, not bytes. For example, the following,

start GeSHi

Type T As Single

Dim array(5) As T = { 32, 43, 66, 348, 112, 0 }
Dim p As T Ptr = @array(0)

While (*p <> 0)
    p += 1
Wend
Print p - @array(0)

end GeSHi

will output "5" regardless of what type T is. This is because there is a five (5) element difference between the first element of array (32) and the element pointed to by p (0).

Specifically, if a and b are both pointers of type T, the distance between them is the number of bytes between them, divided by the size, in bytes, of T, or

`

( cast(byte ptr, a) - cast(byte ptr, b) ) / sizeof(T)

`

See also

• Operator + (Add)
• Operator - (Subtract)
• Operator @ (Address of)
• Operator * (Value of)
• Pointer Operators

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