Re: 'null' references

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What I'm saying is that for code like:

void fn(int* p)
{
WaitForSingleObject(semaphore);
*p++;
ReleaseSemaphore(semaphore);
}

Clearly the above is not exception safe, and if p is NULL or otherwise
invalid to cause an access violation, you will leak semaphore counts. To
say that null and invalid references don't exist suggests that if you
would just change (int* p) to (int& i) the function would be unable to
fail and therefore correctly manages the semaphore, but this is a bogus
argument. The reference version of the function can still throw an access
violation and still needs to be coded for exception safety. This is what
I mean when I say that people who use references to avoid the problems
associated with invalid pointers are misinformed.

According to the standard (8.3.2[4])

"... a null reference cannot exist in a well-defined program, because the
only way to create such a reference would be to bind it to the "object"
obtained by dereferencing a null pointer which causes undefined behaviour".

I understand your point but your own argument is what's "bogus". As soon as
you engage in undefined behaviour then all bets are off. If you define the
above function as taking a reference arg then the undefined behaviour occurs
the moment someone tries to dereference an invalid pointer. That has nothing
to do any function you may be calling at the time. The undefined behaviour
occurs before the function has even started so how can it protect itself.
There is no valid way. Everything might blow up before the function even
begins but the entire environment should be considered corrupt even if it
does start. Your attempt to "protect" the function would therefore be
unreliable but that's beside the point. An "invalid reference" doesn't exist
which is the point. Null pointers do however. So do invalid pointers which
are legal so long as you don't dereference them. The pointer version of the
function would therefore have to protect itself while the reference version
has nothing to protect itself against (since invalid references don't
exist). Even for the pointer version however, it can't protect itself
against an invalid (non-null) pointer using any standard technique noting
that an "access violation" isn't even guaranteed. Nothing is guaranteed and
standard C++ can't even catch these types of so-called "exceptions". A
reference parameter is therefore inherently safe unlike a pointer parameter
since null references don't exist so a reference parameter can never refer
to an invalid object by definition. Pointers legally can however so
dereferning an invalid pointer isn't safe (which is your point) but that has
nothing to do with passing the dereferenced value as a reference arg. I
therefore suggest that you're the one who's misinformed.


.

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