Re: [boost] [smart_ptr] Interest in the missing smart pointer (that can target the stack)

On 2016-01-29 12:40, Rob Stewart wrote:

On January 29, 2016 2:48:56 AM EST, Emil Dotchevski wrote:

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On Thu, Jan 28, 2016 at 7:57 PM, Michael Marcin wrote:

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On 1/28/2016 8:09 PM, Emil Dotchevski wrote:

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And it's way too easy to break that guarantee because it's not the

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semantics that shared_ptr was designed for.

{ foo local; shared_ptr<foo> pl(&local,null_deleter()); .... do_something(p); .... assert(pl.unique()); }

Yes, in the presence of exceptions one must also assert(pl.unique()) in a catch(...), and yes, compile-time errors are better than run-time errors, but I wouldn't sacrifice the availability of weak_ptr and the capacity of shared_ptr to act as THE single smart pointer framework in a program.

Why on earth would you ever do this?

One reason is to get weak_ptr.

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There is no shared ownership semantics here at all.

The point of shared_ptr is to be able to reason that as long as you hold on to a shared_ptr (which you might get by copying another shared_ptr or by locking a weak_ptr), the object will not expire, but you don't hold on to it longer than you need to. This reasoning is perfectly valid within the scope of do_something.

If do_something() saves a copy of the shared pointer in a container, for example, later references will refer to a non-existent object. There's nothing you can do about it short of using assertions or another runtime check with a call to std::terminate() or similar. That's hardly ideal.

I think what Emil describes is a special case of a 'dangling_ptr' idiom, if I may call it that way. The point is that there are cases when object lifetime is controlled by a third party (e.g. the stack state, a foreign library, etc.) and you need a safe way to know when the object has been deleted. So you create a shared_ptr with a null_deleter pointing to that object and save it in that object (or another storage associated with that object). You keep only weak_ptrs to that object in all other places in the code. When you need to use the object you have to lock a weak_ptr and thus check if the object is still alive. The example Emil presented is not quite clear to demonstrate the approach because do_something should be receiving weak_ptr rather than shared_ptr.