[fpc-pascal] Pointer question
bernd.oppolzer at t-online.de
Thu Aug 10 13:43:13 CEST 2023
FWIW, when I added similar functionality to my Stanford Pascal compiler,
I chose not to allow arithmetic
of pointers, but instead I added some functions:
PTRADD (p, i) - p is type ANYPTR, i is integer, result is of type ANYPTR
PTRDIFF (p1, p2) - two pointers, the result is integer
ANYPTR is a predefined type, compatible with every (typed pointer)
ADDR (x) is a function (borrowed from PL/1), which returns an ANYPTR ...
and it is allowed for all types of variables
PTRCAST is the same as PTRADD (p, 0) - and is used to cast pointers
between incompatible pointers (not type safe)
Am 10.08.2023 um 10:52 schrieb Elmar Haneke via fpc-pascal:
>> 1) what does "i := x - x;" do and what is it's purpose and why doesn't "x + x" work the same?
> Subtracting pointers may be useful if they point to consecutive
> memory. The Result is the number of bytes between both addresses.
> Adding pointers is useless, you would get a pointer pointing to some
> address in address space which has no relation to the pointers —
> presumably accessing it would rise an error.
> Therefore, it is a good idea to let the compiler prevent such mistakes.
>> 2) I've used pointer equality of course but what does "x > p" do and what is its purpose?
> It may be useful if pointers do point into a continuous data object,
> e.g. a write-pointer inside a buffer.
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