[LLVMdev] [cfe-dev] Use of Smart Pointers in LLVM Projects

[Jordan Rose]

I don't have much to add here besides +1. I think using std::unique_ptr even for create* functions/methods is the right way to go. Reid's point about an abstraction penalty is interesting, but I don't think we do ownership transfers often enough to actually see a performance hit. (Of course, in the non-transferring case we'd just pass the pointer, not a 'const std::unique_ptr &' or anything.)

Jordan


On Jul 17, 2014, at 16:21 , David Blaikie <dblaikie at gmail.com> wrote:

> There seems to be some uncertainty about the use of smart pointers
> (previously OwningPtr, now std::unique_ptr and std::shared_ptr
> predominantly) in the LLVM project as a whole, so here's a thread to
> discuss/clarify/etc the project preferences/direction with regard to
> smart pointer usage.
> 
> For some context, see discussions in LLVM r212403 and Clang r213307.
> 
> The basic question here seems to be whether smart pointer ownership is
> a direction we want to take the LLVM project in general.
> 
> I've seen others contribute and have myself contributed many patches
> moving towards smart pointer ownership (both in the pre-C++11 days of
> OwningPtr, and much moreso in the post-C++11 world with
> std::unique_ptr and std::shared_ptr being usable inside containers, as
> return values, etc, allowing many more opportunities).
> 
> std::unique_ptr's been used in LLD as far back as r153620.
> std::unique_ptr appeared in LLVM shortly after the C++11 switch with
> Ahmed's work to migrate the project from OwningPtr to std::unique_ptr
> (starting with r202609 and ending with r211259). Originally OwningPtr
> was added in r45261.
> Something in the order of 60 changes across clang and LLVM mention
> unique_ptr in their subject and migrate various APIs to use unique_ptr
> for ownership. Many of which remove uses of explicit delete or helpers
> like DeleteContainerPointers (and removing explicit dtors in many of
> those cases).
> 
> Are people OK with/prefer the use of owning smart pointers in APIs?
> Are there places where you've found them to be too noisy/burdensome
> and would rather use raw pointers or some other abstraction? Would you
> prefer pre-commit review of such changes to adequately consider
> alternatives (such as?)?
> 
> Thanks,
> - David
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