undefined | Better HN

0 pointshvdijk6y ago0 comments

You are allowed to dereference a null pointer, but you are not allowed to access the result or use it as an initialiser for a reference. You can only immediately discard the result, or immediately take the result's address. Using std::addressof(* a) first binds a reference, then uses that reference to take the address, hence the error. You won't get any UBSAN error with &*a.

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3 comments · 1 top-level

MaulingMonkey6y ago· 2 in thread

> You are allowed to dereference a null pointer

Can you cite me something for that? The very first mention of dereferencing in the C++03 standard - ISO/IEC 14882:2003 1.9 Program exection ¶ 4 (page 5) - would seem to disagree:

> Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer). [Note: this International Standard imposes no requirements on the behavior of programs that contain undefined behavior. ]

EDIT: As does 8.3.2 References ¶ 4 (page 136):

> [...] [Note: in particular, 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 behavior. As described in 9.6, a reference cannot be bound directly to a bit-field. ]

hvdijkOP6y ago

The standard doesn't say dereferencing a null pointer is invalid. The fact that it gives it as an example of undefined behaviour is a defect in the standard. In the discussion on core language issue #232, the intent has been explicitly stated:

> Notes from the October 2003 meeting:

> ...

> We agreed that the approach in the standard seems okay: p = 0; *p; is not inherently an error. An lvalue-to-rvalue conversion would give it undefined behavior.

http://open-std.org/JTC1/SC22/WG21/docs/cwg_active.html#232

WRT your edit: no, that says dereferencing a null pointer and binding a reference to the result produces undefined behaviour. That agrees with what I was saying. "which" refers to the whole of "to bind it to the "object" obtained by dereferencing a null pointer", not just to "dereferencing a null pointer".

MaulingMonkey6y ago

I see! Thanks for the reference.

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