Re: references and pointers

From: James Dennett (jdennett_at_acm.org)
Date: 02/23/04 

Date: Sun, 22 Feb 2004 17:52:43 -0800

Sam of California wrote:
> "Artie Gold" <artiegold@austin.rr.com> wrote in message
> news:c0f5l9$15q5j5$1@ID-219787.news.uni-berlin.de...
>
>>A reference cannot be NULL (so you don't have to check for that to
>>program robustly).
>
>
> So is the following invalid?

Your code is "invalid" -- specifically, it invokes
undefined behavior by referencing a null pointer.

> My VC 6 compiler accepts it; the value it
> outputs is "00000000" for "&a".

VC6 is quite allowed to do that, or to print
"error in program", or to do anything else.

>
> // - - - - - - - - -
> void Referrence(int &a) {
> cout << "Address: " << &a << '\n';
> return;
> }

That's a perfectly reasonable function, modulo
the spelling mistake in its name, which will
write the address of an int to cout (presumably
a stream, most likely ::std::cout).

> // - - - - - - - - -
>
> int main(int argc, char* argv[], char *envp[]) {

Note that the envp part of this is a non-standard
extension, and not really needed; std::getenv can
be used to read from the environment.

> int &r = *((int *)NULL);

There you dereference a null pointer, which is not
permitted. Anything goes.

> Referrence(r);
> return 0;
> }

Hope this clarifies -- there is no way to get a "null"
reference in a valid program. By the time your check
for a "null" reference might catch something, it's too
late.

-- James.

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