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Re: NULL pointer arithmetic issues

On 24.02.2020 23:35, Mouse wrote:
>> Unless I remember wrong, older C standards explicitly say that the
>> integer 0 can be converted to a pointer, and that will be the NULL
>> pointer, and a NULL pointer cast as an integer shall give the value
>> 0.
> The only one I have anything close to a copy of is C99, for which I
> have a very late draft.
> Based on that:
> You are not quite correct.  Any integer may be converted to a pointer,
> and any pointer may be converted to an integer - but the mapping is
> entirely implementation-dependent, except in the integer->pointer
> direction when the integer is a "null pointer constant", defined as
> "[a]n integer constant expression with the value 0" (or such an
> expression cast to void *, though not if we're talking specifically
> about integers), in which case "the resulting pointer, called a null
> pointer, is guaranteed to compare unequal to a pointer to any object or
> function".  You could have meant that, but what you wrote could also be
> taken as applying to the _run-time_ integer value 0, which C99's
> promise does not apply to.  (Quotes are from
> I don't think there is any promise that converting a null pointer of
> any type back to an integer will necessarily produce a zero integer.
> /~\ The ASCII				  Mouse
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$ cat test.cpp

#include <cstddef>

main(int argc, char **argv)
	if (((char *)0)[argc])
		return 1;
		return 0;

$ g++ test.cpp
$ ./a.out
Memory fault (core dumped)

And some variations:

$ g++ test.cpp
test.cpp: In function ‘int main(int, char**)’:
test.cpp:6:15: warning: converting NULL to non-pointer type
  if (NULL[argc])
test.cpp:6:15: error: invalid types ‘long int[int]’ for array subscript

$ g++ test.cpp
test.cpp: In function ‘int main(int, char**)’:
test.cpp:6:18: error: invalid types ‘std::nullptr_t[int]’ for array
  if (nullptr[argc])

NULL in C is expected to be harmonized with nullptr from C++.

We still can store NULL/nullptr in variables as before and there is no
change in the produced code. The only change is on the syntax level as
we can catch more bugs earlier. Whenever a compiler will be smart enough
to deduce that the code is nullptr[0] it will raise an error.

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