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744. What is the lifetime of an exception pointed to by anexception_ptr?

Section: 17.9.7[propagation]Status:CD1Submitter: Alisdair MeredithOpened: 2007-10-10Last modified: 2016-01-28

Priority:Not Prioritized

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Discussion:

Without some lifetime guarantee, it is hard to know how this type can beused. Very specifically, I don't see how the current wording wouldguarantee andexception_ptr caught at the end of one thread could be safelystored and rethrown in another thread - the original motivation for thisAPI.

(Peter Dimov agreed it should be clearer, maybe a non-normative note toexplain?)

[Bellevue:]

Agree the issue is real.

Intent is lifetime is similar to a shared_ptr (and we might even want toconsider explicitly saying that it is a shared_ptr< unspecified type >).

We expect that most implementations will use shared_ptr, and thestandard should be clear that the exception_ptr type is intended to besomething whose semantics are smart-pointer-like so that the user doesnot need to worry about lifetime management. We still need someone todraught those words - suggest emailing Peter Dimov.

Move to Open.

Proposed resolution:

Change 17.9.7[propagation]/7:

-7- Returns: Anexception_ptr object that refers to the currentlyhandled exception or a copy of the currently handled exception, or anullexception_ptr object if no exception is being handled.The referenced object remains valid at least as long as there is anexception_ptr that refers to it.If the function needs to allocate memory and the attemptfails, it returns anexception_ptr object that refers to an instance ofbad_alloc. It is unspecified whether the return values of two successivecalls tocurrent_exception refer to the same exception object. [Note:that is, it is unspecified whethercurrent_exception creates a new copyeach time it is called.--end note]