Exactly!

> This advice doesn't seem quite right to me, and in my codebases I strictly forbid passing shared_ptr by const reference

There is at least one use case I can think of: the function may copy the shared_ptr, but you want to avoid touching the reference count for the (frequent) case where it doesn't. This is an edge case, though, and personally I almost never do it.

Otherwise, I'm inclined to agree -- don't pass around smart pointers unless you're actually expressing ownership semantics. Atomics aren't free, ref-counting isn't free, but sometimes that genuinely is the correct abstraction for what you want to do.

One more point: shared ownership should not be used as a replacement for carefully considering your ownership model.

(For readers who might not be as familiar with ownership in the context of memory management: ownership is the notion that an object's lifetime is constrained to a given context (e.g. a scope or a different object -- for instance, a web server would typically own its listening sockets and any of its modules), and using that to provide guarantees that an object will be live in subcontexts. Exclusive ownership (often, in the form of unique_ptr) tends to make those guarantees easier to reason about, as shared ownership requires that you consider every live owning context in order to reason about when an object is destroyed. Circular reference? Congrats, you've introduced a memory leak; better break the cycle with weak_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; };

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.

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.

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.)

My experience is the opposite. It has to do with the coarseness of the objects involved and the amount of inter-object links. We typically have a vast variety of classes. Many of them have shared_ptr members, resulting in rich graphs.

Many methods capture the shared_ptr parameters by copying them inside other objects. However, many methods just want to call a couple methods on the passed-in object, without capturing it. By standardizing on const shared_ptr &, all calls are alike, and callees can change over time (e.g. from not capturing to capturing.)

foo(std::shared_ptr<bar>) is copy-constructed as part of your function call (bumping the refcount) unless copy elision is both available and allowed. It's only ideal if you almost always pass newly instantiated objects.

Pass by const reference is the sweet spot. If you absolutely must minimize the refcount bumps, overload by const reference and by rvalue.

As for shared_ptrs being very rare, uh, no. We use them by the truckload. To each their own!

What?

invariably it's more like when) you later decide to share ownership,

shared_ptr shouldn't even be necessary for keeping track of single threaded scope based ownership.

As for shared_ptrs being very rare, uh, no. We use them by the truckload. To each their own!

You might want to look into that, you shouldn't need to count references in single threaded scope based ownership. If you need something to last longer, make it's ownership higher scoped.

If something already works it works, but this is not necessary and is avoiding understanding the actual scope of variables.

The context you're missing is in your post's GP. That poster holds a std::shared_ptr<bar> (for whatever perfectly valid reason) and wishes to pass it to foo(). However, he declares it as foo(const bar &) because the callee does not need to share in the ownership of the shared_ptr. That means it gets called as foo(*p.get()).

> scope based ownership

That's the incorrect assumption that you came to. Obviously if bar is only used for stack-based scope variables, no shared_ptr is needed.

I didn't miss any of that, that exactly what I thought it meant. I just don't know what you mean by precisely wish to bar ownership

That's the incorrect assumption that you came to.

Prove it. In a single threaded program with scope based ownership, that shared_ptr is going to be freed somewhere, so why not just have it exist in that scope as a unique_ptr so the ownership scope is clear?

Obviously if bar is only used for stack-based scope variables, no shared_ptr is needed.

Are you saying I'm wrong then saying the exact thing I just said?

Scope based ownership would be a unique_ptr that frees the heap memory when it goes out of scope (if it isn't moved).

This doesn't make sense. Why would a function in a more narrow scope need to take or share ownership? The variable passed will persist before and after the function call.

The incorrect assumption that you came to is that we were talking about stack variables.

I don't know what this means here. I said scope, you are saying stack. If lifetime is being dictated purely by scope, you don't need shared_ptr, you can just use a unique_ptr at the correct scope.

But anyways, here's an example that's both scope-based and single threaded: std::vector<std::shared_ptr<bar> > v;

This isn't scope based lifetime, this is lifetime based on some other criteria than going out of scope. I will show you with what you wrote:

and another that duplicates and removes elements according to some ongoing criteria.

What if the callee sometimes wants to get a reference count and sometimes doesn't? In the latter case, your proposed signature forces an unnecessary pair of atomic reference count operations. But if you use

    foo(bar const&)

You could stick std::enable_shared_from_this` under `bar`. But `std::enable_shared_from_this` adds a machine word of memory, so you might not want to do that.

If you pass

    foo(shared_ptr<bar> const&)

    foo(bar const&, shared_ptr<bar> const&)

You can win actually. Just use https://www.boost.org/doc/libs/1_85_0/libs/smart_ptr/doc/htm... or your favorite intrusive reference-counted smart pointer, not `std::shared_ptr`. If you do, you get the same capabilities that `std::enable_shared_from_this` grants but without any of the downsides.

   > foo(shared_ptr<bar> const&)

Actually this is usually not the case (assuming of course that caller is holding the original pointer in a shared_ptr<bar> which is the use case we were discussing.)

That shared_ptr<bar> instance is held either on the stack (with address FP + offset or SP + offset) or inside another object (typically 'this' + offset.) To call foo(const shared_ptr<bar> &), the compiler adds the base pointer and offset together, then passes the result of that addition - without dereferencing it.

So as it turns out, you may actually have one fewer pointer chase in the const shared_ptr<bar> & case. For example, if foo() is a virtual method and a specific implementation happens to ignore the parameter, neither the caller nor the callee ever dereference the pointer.

The one exception is if you've already resolved the underlying bar& for an unrelated reason in the caller.

I do agree that intrusive_ptr is nice (and we actually have one codebase that uses something very similar.) However shared_ptr has become the standard idiom, and boost can be a hard sell engineering-wise.

You're overthinking it. Think in cache lines. No matter what the instructions say, with all their fancy addressing modes, foo has to load two cache lines: one holding the shared_ptr and another holding the pointee data. If we instead passed bar* in a register, we'd need to grab only one cache line: the pointee's.

Sure. Maybe the caller already has a fully formed shared_ptr around somewhere but not in cache. Maybe foo often doesn't access the pointee. But how often does this situation arise?

The cache doesn't make a difference here. To clarify: we start with a shared_ptr<bar> instance. It must get dereferenced to be used. It must either be dereferenced by the caller (the const bar & contract) or by the callee (the const shared_ptr<bar> & contract).

If the caller dereferences it, it might turn out to be superfluous if the callee wasn't actually going to use it. In this case const shared_ptr<bar> & is more efficient.

However, if the caller happened to have already dereferenced it prior to the call, one dereferencing would be avoided. In this case const bar & is more efficient.

> Sure. Maybe the caller already has a fully formed shared_ptr around somewhere but not in cache.

This is where our misunderstanding is. The caller starts out by only having a shared_ptr. Someone (caller or callee) has to dig the bar * out.