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by worstspotgain 703 days ago
Let me preface by noting that I don't necessarily disagree with any part of what you wrote. However, there are design patterns that exceed the guardrails you're thinking of, and those are the patterns that benefit the most from shared_ptr.

Typically, they involve fine- to medium-grained objects, particularly those that have dynamic state (meaning by-value copies are not an option.)

An example might be a FlightAware-like system where each plane has a dynamically-updated position:

   class Plane { ... void UpdatePosition(const Pos &); Pos GetPosition() const; };
   using PlanePtr = std::shared_ptr<Plane>;
   using PlaneVec = std::vector<PlanePtr>;
   class Updater { ... PlaneVec mPlanes; };
   class View { ... PlaneVec mPlanes; };
Updater routinely calls UpdatePosition(), whereas View only calls const methods on Plane such as GetPosition(). There can be a View for, say, Delta flights and one for United. Let's simplify by assuming that planes are in the sky forever and don't get added or removed.

Destructing Updater doesn't affect Views and vice-versa. Everything is automatically thread-safe as long as the Pos accesses inside each Plane are thread-safe.

The key here is that Plane is fine-grained enough and inconsequential enough for lazy ownership to be ideal.
1 comments
vitus 703 days ago
> Let's simplify by assuming that planes are in the sky forever and don't get added or removed.

If planes are around forever, wouldn't you be better off interning them? e.g. having a single global std::vector<Plane> (or std::array<Plane, N>) and passing around offsets in that array? And your PlaneVec would just be a glorified std::vector<size_t> (or int)? I don't see any value in maintaining a reference count if you're never intending to clean up these objects.

(The argument for using int here would be if you always have fewer than 2 billion planes, and so you can store a PlaneVec in less space. size_t is indistinguishable from Plane* in this context; you have the same amount of indirections either way.)

As I said, shared ownership has its uses, but most instances I've seen could have been replaced with a different model and would have been less painful to debug memory leaks and use-after-free.

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worstspotgain 703 days ago
> If planes are around forever, wouldn't you be better off interning them? e.g. having a single global std::vector<Plane> (or std::array<Plane, N>) and passing around offsets in that array? And your PlaneVec would just be a glorified std::vector<size_t> (or int)? I don't see any value in maintaining a reference count if you're never intending to clean up these objects.

Definitely not. :) I added that restriction just to sidestep the need to add and remove planes to/from existing Updaters and Views. Besides, you might have an Updater for US flights and one for European ones, and a View might span worldwide Delta flights or just US ones. Updaters and Views might come and go dynamically. The reference counting is key.

In this example, it doesn't matter when Planes get cleaned up, but it does matter that they do. A better alternative than the one you're proposing would be to just use vector<Plane *> and leak the planes, but that's crappy for different reasons (e.g. long-term memory usage and it would bar Plane from, say, RAII-owning its own log file.)

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