Understanding exception safety of shared_ptr::void reset (U* p);

Question

I'm trying to understand the exception safety of the template <class U> void shared_ptr::reset (U* p); function. In the documentation I found this:

Additionally, a call to this function has the same side effects as if shared_ptr's destructor was called before its value changed (including the deletion of the managed object if this shared_ptr was unique).

So, we can assume that destructor doesn't throw. But what if the operator new throws? What state of the shared_ptr will we have then? Does it remain unchanged?

[This](http://en.cppreference.com/w/cpp/memory/shared_ptr/reset) is a little more descriptive: " std::bad_alloc if required additional memory could not be obtained. May throw implementation-defined exception for other errors. delete ptr is called if an exception occurs." — AndyG, Nov 17 '15 at 04:07
Which `operator new`? The one called to allocate the new control block? — T.C., Nov 17 '15 at 04:08
As a small sidenote preferr http://cppreference.com over cplusplus.com. [Cplusplus.com has many small mistakes and some bad examples.](http://stackoverflow.com/questions/6520052/whats-wrong-with-cplusplus-com) — RedX, Nov 17 '15 at 04:12

score 2 · Accepted Answer · answered Nov 17 '15 at 04:13

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Per [util.smartptr.shared.mod], all four overloads of shared_ptr::reset(stuff) are exactly equivalent to

shared_ptr(stuff).swap(*this)

If constructing shared_ptr(stuff) throws (e.g., if allocating the new control block (or whatever equivalent mechanism used by the implementation) throws), then *this is unaffected because you never get to the swap, and any pointer passed in stuff is deleted appropriately (because that's guaranteed by shared_ptr's constructor).

swap itself is nothrow, and so is destroying the temporary shared_ptr after the swap.

answered Nov 17 '15 at 04:13

T.C.

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Great answer. I might add that the destructor for `stuff` must also be `noexcept` – AndyG Nov 17 '15 at 04:15
@AndyG That's required by the corresponding constructor's spec, on pain of UB. – T.C. Nov 17 '15 at 04:16

score 1 · Answer 2 · answered Nov 17 '15 at 04:08

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The function takes a pointer. That means the new would have to be done prior to execution entering the function. Inside the function I assume only a pointer and some of integer type are being assigned and those don't throw.

answered Nov 17 '15 at 04:08

Anon Mail

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Shared pointer should allocate internal storage (with counter). – kriomant Nov 17 '15 at 04:14

Understanding exception safety of shared_ptr::void reset (U* p);

2 Answers2