[cfe-dev] Default arguments considered harmful?

[Kaelyn Takata]

On Fri, May 2, 2014 at 10:42 AM, David Blaikie <dblaikie at gmail.com> wrote:

> On Fri, May 2, 2014 at 9:40 AM, Philip Reames <listmail at philipreames.com>
> wrote:
> >
> > On 05/02/2014 02:18 AM, David Chisnall wrote:
> >>
> >> On 2 May 2014, at 09:16, David Tweed <david.tweed at arm.com> wrote:
> >>
> >>> I wonder if, with C++11 named initialization syntax for PODs, something
> >>> could be done so that in cases like this a (function specific, I guess)
> >>> "optional options object" could be used. (This would avoid one of the
> big
> >>> problems with C++ optional arguments, which is that if one towards the
> >>> end
> >>> needs to be set to a non-default value all the preceding options need
> >>> setting.) That would certainly make things a lot easier to read in
> cases
> >>> like these.
> >>
> >> The related issue is that they complicate overloading.  I've been
> bitten a
> >> few times in clang code by the compiler deciding that, when I specified
> a
> >> pointer instead of a reference for a parameter, it would happily coerce
> it
> >> to a bool and then use the default values for the rest of the
> parameters,
> >> giving some very strange results - no errors, no warnings, just the code
> >> doing something unexpected.
> >>
> >> David
> >>
> > David, I think you're pointing out a slightly different issue... one I've
> > also been bitten by.
> >
> > Does anyone know of a legitimate reason to have an implicit conversion to
> > bool as a function argument?  Maybe we should add an optional warning for
> > this case in particular?
>
> My suspicion is that this would (just barely) not meet our bar for
> false positive rate. (my bet, though it's not a sure thing, is that we
> do this in the LLVM project at least tens of times intentionally (ie:
> not a bug, but subtle code))
>
> But, arguably, rewriting the argument from "func(x)" to "func(x !=
> nullptr)" is a benefit in readability and sometimes we're OK with a
> few more false positives if they're just "badly written/confusing
> code"
>

I feel like this warning would be quite useful/valuable if it is (or can
be) limited to implicit bool conversions for optional arguments.  Though
like you, I suspect warning on implicit bool conversions even for required
arguments may not fall under the threshold for false positives.

Cheers,
Kaelyn


> - David
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