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

[Andrew Trick]

On May 15, 2014, at 3:22 AM, Artyom Skrobov <Artyom.Skrobov at arm.com> wrote:

> Hello,
> 
> Carrying on this conversation from llvm-commits:
> 
>>>>> Would it be reasonable if we name both SwapByteOrder() -- it's
> difficult
>>>>> to describe their purpose in any other way -- and make the in-place
>>>>> function take a pointer, instead of a reference?
>>>> 
>>>> Pointer is the wrong API: it implies having to check for null.
>>> 
>>> I see that in general, the choice between pointer parameters and
> reference
>>> parameters can mean one of many things: input vs output, change of
> ownership
>>> vs no change, validity of NULL, and perhaps more.
>>> 
>>> For example, Google C++ Style Guide mandates: "All parameters passed by
>>> reference must be labeled const. [...] it is a very strong convention in
>>> Google code that input arguments are values or const references while
> output
>>> arguments are pointers."
> (http://google-styleguide.googlecode.com/svn/trunk/
>>> cppguide.xml#Reference_Arguments )
>> 
>> I haven't seen much of that around here.
>> 
>>> I see that LLVM Coding Standards document doesn't touch this subject at
> all.
>>> Should we use this opportunity to add to it that in LLVM, the choice
> between
>>> pointer parameters and reference parameters is defined by whether NULL is
> a
>>> valid input?
>> 
>> Not sure that's necessary, but feel free to send a proposal to llvmdev.
> 
> What does the community think about such an addition?
> 
> ===================================================================
> --- docs/CodingStandards.rst	(revision 208684)
> +++ docs/CodingStandards.rst	(working copy)
> @@ -837,6 +837,21 @@
>     It's okay to put extra implementation methods in a public class itself.
> Just
>     make them private (or protected) and all is well.
> 
> +Use References for Non-null Function Arguments
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +Using a pointer for a function argument implies that the function must
> treat
> +NULL input in a sensible way. Where such check is unpractical (e.g. when
> every
> +call site is known to pass a non-null input), or when you want to express
> it in
> +the function signature that nullptr is not a valid input, use a reference
> +argument instead.
> +
> +This is an application of "Use references when you can, and pointers when
> you
> +have to." maxim from the C++FAQ. Avoid using the pointers vs. references
> +distinction to convey other meanings, e.g. to mark the distinction between
> +input and output arguments, as may be advised by other style guides.
> +
> +
> .. _early exits:
> 
> Use Early Exits and ``continue`` to Simplify Code
> ===================================================================


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.

-Andy