On transferring or copying ABI pointers between smart pointers

Raymond Chen

A customer traced a crash to a reference count underflow. But they were using smart pointers. I thought smart pointers avoided reference count problems.

Smart pointers make it easier to avoid reference count problems, but you still have to use them correctly.

There are some basic operations for ABI interop with smart pointers.

  • Take ownership: Given a raw pointer, accept the pointer and assume responsibility for releasing it. This typically goes by the name “attach”.
  • Share ownership: Given a raw pointer, accept the pointer and share ownership by incrementing the reference count. This doesn’t have a standard name, but I’ll call it “copy” for the purpose of this discussion.
  • Non-destructive access: Given a smart pointer, produce the raw pointer without relinquishing ownership. This is typically called “get”.
  • Abandon ownership: Given a smart pointer, produce the raw pointer and relinquish ownership. This is typically called “detach”.

It is important that when you move raw pointers between smart pointers, that you match up the two semantics: If you want to transfer ownership, then the donor should use detach semantics and the recipient should use attach semantics. If you want to share ownership, then the donor should use get semantics and the recipient should use copy semantics.

If you mess up, then you end up with reference count bugs: If the donor detaches but the recipient copies, then you have a reference count leak. If the donor offers with “get” but the recipient attaches, then you have an over-release.

Here are some tables showing various Windows smart pointer libraries and how they express the two pairs of operations. Note that this table is just an overview; consult the corresponding documentation for further information. For example, some of the methods require that the smart pointer be non-null.

First, the attach/detach (transfer) operations:

Library Detach (donor) Attach (recipient)
_com_ptr_t sp.Detach() sp.Attach(p)
_com_ptr<T>(p, false)
ATL (CComPtr) sp.Detach() sp.Attach(p)
MFC (IPTR) sp.Detach()
/* Note 1 */
sp.Attach(p)
sp.Attach(p, FALSE)
IPTR(T)(p, FALSE)
WRL sp.Detach() sp.Attach(p)
wil (com_ptr) sp.detach() sp.attach(p)
C++/WinRT (com_ptr) sp.detach()
detach_abi(sp)
sp.attach(p)
attach_abi(sp, p)
com_ptr<T>(p, take_ownership_from_abi)

Note 1: IPTR’s Detach() method does not return the raw pointer.

And then the get/copy (share) operations:

Library Get (donor) Copy (recipient)
_com_ptr_t static_cast<T*>(sp)
sp.GetInterfacePtr()
sp = p
sp.Attach(p, true)
_com_ptr<T>(p, true)
ATL (CComPtr) static_cast<T*>(sp)
*sp
sp.p
sp = p
CComPtr<T>(p)
MFC (IPTR) static_cast<T*>(sp)
sp.GetInterfacePtr()
sp = p
sp.Attach(p, TRUE)
IPTR(T)(p)
IPTR(T)(p, TRUE)
WRL sp.Get() sp = p
ComPtr<T>(p)
wil (com_ptr) sp.get() sp = p
com_ptr<T>(p)
C++/WinRT (com_ptr) sp.get()
get_abi(sp)
sp.copy_from(p)

Of course, if you are remaining within one row of the table, then you can usually avoid having to operate through ABI pointers. For example, you can just use sp1 = sp2 to copy one smart pointer to another smart pointer of the same type, or sp1 = std::move(sp2) to transfer ownership. The purpose of the above tables is to help you move between rows: The donor and recipient should both be attach/detach (transfer) semantics, or they should both be get/copy (share) semantics.

But wait, there’s another option, which I will call the “recipient” pattern.

Library Create recipient Transfer to
recipient
Copy to
recipient
_com_ptr_t &sp *r = sp.Detach()  
ATL (CComPtr) &sp
&sp.p
*r = sp.Detach() sp.CopyTo(r)
MFC (IPTR) &sp  
WRL &sp
sp.GetAddressOf()
sp.ReleaseAndGetAddressOf()
*r = sp.Detach() sp.CopyTo(r)
wil (com_ptr) &sp
sp.addressof()
sp.put()
sp.put_void()
*r = sp.detach() sp.query_to(r)
sp.copy_to(r)
C++/WinRT (com_ptr) sp.put()
sp.put_void()
put_abi(sp)
*r = sp.detach()
*r = detach_abi(sp)
sp.copy_to(r)

The “recipient” pattern produces a T**, and it’s up to the donor to decide whether to transfer or copy ownership to it. This pattern is used by most of COM: For example, Create­Stream­On­HGlobal takes a recipient as its final parameter, and it puts a reference to the newly-created stream in that recipient. You as the caller don’t know or care whether the function copied or transferred a reference to the stream into your recipient pointer; all that you care about is that when the function returns, your recipient pointer received a reference to the thing.

Bonus chatter: C++ shared_ptr and unique_ptr have similar patterns and pitfalls. For example, given the declarations unique_ptr<T> u1, u2;, you shouldn’t write things like u1.reset(u2.get()) or std::shared_ptr<int>(u1.get()); since they result in double-ownership and therefore will eventually result in double-destruction.

Bonus reading: We’re using a smart pointer, so we can’t possibly be the source of the leak.

3 comments

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  • Paulo Pinto 0

    Another good example of the pain that is to deal with COM during the decades, with multiple library reboots, and those examples are missing the interop issues with .NET and VB6, on the same context of transfering ownership.

    One would expect the tooling to have improved, with so many approaches on how to do COM from C++.

  • Markus Schaber 0

    There’s the advantage of modern languages like Rust: They have the same power, but with simpler syntax (because it hasn’t been bolted into the language as an afterthought), and the compiler guarantees that you cannot make those mistakes, because ownership and transferring it are concepts of the language, not just convention.

  • Neil Rashbrook 0

    In the smart pointer library that I’m most used to, the default is to get (via a cast operator) and copy, and if you want to detach, you end up with a templated type which when assigned to the smart pointer automatically attaches. There’s also a method to detach into a recipient, which is the preferred way to populate one, although one of the ways you can still mess things up is by assigning it manually. There is no address-of operator, but there is a global template class which takes a smart pointer and has a cast operator that returns a recipient.

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