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2337. shared_ptr operator*() should not be noexcept

Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: NAD Submitter: Stephan T. Lavavej Opened: 2013-10-05 Last modified: 2017-07-17

Priority: 2

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Discussion:

20.3.1.3.5 [unique.ptr.single.observers]/3: "pointer operator->() const noexcept; Requires: get() != nullptr."

20.3.2.2.6 [util.smartptr.shared.obs]/2: "T& operator*() const noexcept; Requires: get() != 0."

20.3.2.2.6 [util.smartptr.shared.obs]/5: "T* operator->() const noexcept; Requires: get() != 0."

Narrow-contract functions should not be noexcept.

[2014-02-15 Issaquah]

Issue is contentious, raise to P2.

[2015-02 Cologne]

AM: This ship has sailed. JM: What's the issue? AM: operator-> has narrow contract and should never have had noexcept. DK: Not quite. We explicitly called out that for shared_ptr this is fine. You said so in your "narrow contract" paper. GR: This would be a fairly major regression in the design of {unique,shared}_ptr over raw pointers; raw pointer dereferencing is noexcept. It's not a performance regression but a usability regression. AM: Do we expect users to query noexpect on dereference expressions? Room: Yes. VV: We don't just expect it, we have seen it. JM: Yes, users may be querying something like noexcept(x->y) and expect to be checking y, but silently end up checking x->.

Close as NAD, with explanation from GR.

This wording is relative to N3691.

1. In 20.3.1.3 [unique.ptr.single]/1, class template unique_ptr synopsis for single objects, change as indicated:

   pointer operator->() const noexcept;
   

2. In 20.3.1.3.5 [unique.ptr.single.observers] change as indicated:

   pointer operator->() const noexcept;
   

   -3- Requires: get() != nullptr.

   -4- Returns: get().

   -?- Throws: Nothing.

   -5- Note: use typically requires that T be a complete type.

3. In 20.3.2.2 [util.smartptr.shared]/1, class template shared_ptr synopsis, change as indicated:

   T& operator*() const noexcept;
   T* operator->() const noexcept;
   

4. In 20.3.2.2.6 [util.smartptr.shared.obs] change as indicated:

   T& operator*() const noexcept;
   

   -2- Requires: get() != 0.

   -3- Returns: *get().

   -?- Throws: Nothing.

   -4- Remarks: When T is void, it is unspecified whether this member function is declared. If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well formed.

   T* operator->() const noexcept;
   

   -5- Requires: get() != 0.

   -6- Returns: get().

   -?- Throws: Nothing.

[2015-03-03, Geoffrey provides rationale]

Rationale:

It is by design that these members are noexcept, and changing that now would be a substantial regression in functionality. These classes were designed to substitute for plain pointers as transparently as possible, so since those operations are effectively noexcept on plain pointers, they should be noexcept on unique_ptr and shared_ptr as well. This matters in practice because we expect these members to be used fairly often inside the noexcept operator, and such code could be broken by this change. These design considerations override our general policy against noexcept for narrow-contract functions.

It is notable that N3279, which proposed this policy, did not propose striking noexcept from these operations. It's not clear if the omission of operator* and operator-> was an oversight, or an intentional reflection of the above considerations. N3279 was based on N3248 by the same authors, which states that:

"Most applications of noexcept for unique_ptr and shared_ptr are on functions with wide contracts. However, there are preconditions on the atomic access functions, so these should lose the specification."

Proposed resolution: