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### 707. null pointer constant for `exception_ptr`

Section: 17.9.7 [propagation] Status: NAD Submitter: Jens Maurer Opened: 2007-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

From the Toronto Core wiki:

What do you mean by "null pointer constant"? How do you guarantee that `exception_ptr() == 1` doesn't work? Do you even want to prevent that? What's the semantics? What about `void *p = 0; exception_ptr() == p`? Maybe disallow those in the interface, but how do you do that with portable C++? Could specify just "make it work".

Peter's response:

null pointer constant as defined in 7.3.12 [conv.ptr]. Intent is "just make it work", can be implemented as assignment operator taking a unique pointer to member, as in the unspecified bool type idiom.

[ Bellevue: ]

> Original implementation was possible using the "unspecified-null-pointer" idiom, similar to unspecified-bool.

> Even simpler now with nullptr_t.

> NAD Rationale : null pointer constant is a perfectly defined term, and while API is clearly implementable there is no need to spell out implementation details.

Proposed resolution: