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

Chris Lattner clattner at apple.com
Mon May 26 20:45:34 PDT 2014

On May 26, 2014, at 8:21 PM, Chandler Carruth <chandlerc at google.com> wrote:

> 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.
> Certainly.
> My suggestion: unless we have evidence this is confusing people trying to contribute to LLVM, then it isn't broken as is.

Chandler, Andy's recollection is correct.

I was personally infatuated by this idea and pushed it through a ton of the LLVM IR APIs (this was many years ago, (in the pre-1.0 days, and then again early in Clang's development because I'm a slow learner).  It led to all sorts of weird cases where some arguments to IR objects would be taken by reference and some by pointer.  It made it impossible to remember whether a given function took a pointer or a reference, and made for absolute madness trying to program against the APIs.  It got to the point where literally you had to do a build and just add the missing &/*'s to get the build to go through.


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