[LLVMdev] Guidance on using pointers vs. references for function arguments

David Blaikie dblaikie at gmail.com
Mon May 26 19:59:13 PDT 2014


On Mon, May 26, 2014 at 7:41 PM, Andrew Trick <atrick at apple.com> wrote:
>
> On May 26, 2014, at 5:02 PM, Chandler Carruth <chandlerc at google.com> wrote:
>
>
> On Mon, May 26, 2014 at 4:43 PM, Andrew Trick <atrick at apple.com> wrote:
>>
>> This has been discussed before but I can’t find a reference to it. I could
>> have sworn this was in the coding convention at some point. Here’s what I
>> remember: during early LLVM development there was an effort to establish the
>> convention that you described above—use pointer types only when nullptr is
>> valid. This led to a lot of redundant declarations and annoying taking of
>> addresses and dereferences. It turns out that the convention doesn’t really
>> help for most informal/internal APIs. It’s actually no harder to debug a
>> SIGSEGV than a nullptr check. I also adhered to this convention in a
>> previous project and it never paid off.
>>
>> Once you begin working on a piece of code you get a feel for which types
>> should be passed as pointers and which should be passed as reference. Then
>> you try to pass types consistently regardless of whether a null input is
>> valid. For example, some types, like the current context, should never be
>> copied or passed by value and are obviously not null. That’s lower overhead
>> in practice forcing callers to convert to a reference whenever we want to
>> skip a null check.
>
>
> This last sentence should read: it’s lower mental overhead for the
> programmer to use the same type consistently rather than worrying about
> another convention to follow at every call.
>
> I would personally be happy to follow the pointer may be nullptr convention
> if it were used consistently. I was just trying to reiterate arguments
> against it that I’d seen w.r.t LLVM codebase, and I don’t see much value in
> forcing the convention everywhere.

Yeah, there's certainly some local consistency that's not worth
overriding just yet - but we do the same thing with function names,
for example (we've still got lots of functions named UpperFirst, try
to keep a single class's member functions consistent amongst
themselves, but when creating new classes we prefer the new convention
of lowerFirst).

But when given the choice (because there's no local consistency to
worry about) I'll pass by reference where possible & suggest others do
the same - but I'm not fussed enough to push this to be in the LLVM
Style guide. Won't complain if it ends up there either, though :)

- David

>
> -Andy
>
> FWIW, I disagree.
>
> I much prefer to pass by reference unless there is the expectation of null
> inputs. I have never really agreed with the complaints about taking the
> address and have never found it to be a burden. I also find the simplicity
> of a consistent rule far more appealing than "getting a feel for which
> types" should be passed as pointers.
>
> Ironically, using a reference can result in better optimizations by deleting
> redundant null checks. While I certainly hope this isn't relevant to the
> performance of LLVM, it's still not something to completely disregard.
>
> Anyways, I've never really seen this become a problem in practice. I'm
> pretty happy for folks to use whatever local conventions they want. I'm
> usually happy to switch from reference to pointer if I'm hacking some part
> of the codebase I don't usually touch and one of the maintainers really
> prefers one over the other. It's not a big deal either way.
>
>
>
>
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