Re: Null references.

James Kanze <>
Wed, 16 Jul 2008 02:23:23 -0700 (PDT)
On Jul 16, 5:04 am, "Alf P. Steinbach" <> wrote:

There are some exceptions to incurring UB for dereferencing a
nullpointer. The most important one is that any given compiler
vendor can decide to give that a well-defined meaning at
compile time for that compiler, and exploit that to implement
various macros in the standard library. Another one is the
context of a typeid call.

Hows that?

Modulo vendor specific guarantees (of course), it's undefined
behavior to dereference a null pointer in code that actually
gets executed. Something like sizeof( *p ) is fine, even if p
is a null pointer. But typeid normally requires execution,
e.g.: typeid( *p ) is undefined behavior if p is a null pointer
(and the pointed to object has a polymorphic type, but then,
that's the usual reason for using typeid).

James Kanze (GABI Software)
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