On 3/29/22 6:55 AM, Timur Doumler via Std-Proposals wrote:
> I don’t agree that the second example is bizarre. In fact, I had this
> exact case in a real-world codebase not too long ago. The idea was
> that you start an audio device and pass it a process callback (which
> is mandatory) as well as “device started” and device stopped”
> callbacks, which are optional callbacks that the device will call
> before it starts processing and after it stopped processing so you can
> allocate/deallocate resources required for processing. It would be
> very nice to be able to write
>
> void start_device(auto process_callback, auto started_callback = []{},
> auto stopped_callback = []{});

Do you really want templates in that case? It seems to me that
std::function_ref as proposed in P0792 <https://wg21.link/p0792> / P2472
<https://wg21.link/p2472> provides a better solution here.

Tom.

>
> so that if someone doesn’t want to pass the optional two callbacks,
> they can just leave them out. And to let the function template deduce
> the type of all three.
>
> In practice the signature looked a bit more complicated as there are
> constraints on the function signature of each callback, but the idea
> is the same.
>
> Cheers,
> Timur
>
>> On 29 Mar 2022, at 01:39, Jason McKesson via Std-Proposals
>> <std-proposals_at_[hidden]> wrote:
>>
>> On Mon, Mar 28, 2022 at 3:07 PM Kilian Henneberger via Std-Proposals
>> <std-proposals_at_[hidden]> wrote:
>>>
>>> Hello,
>>>
>>> I am wondering whether default arguments should participate in template
>>> argument deduction.
>>> Currently they don't. Therefore, given following function templates:
>>>
>>> template<class Pred>
>>> void sort(Pred p = std::less{});
>>>
>>> void set_callback(auto cb = []{});
>>>
>>> auto create_logger(auto sl = std::source_location::current());
>>>
>>> Calling any of those without providing function arguments, will fail to
>>> compile.
>>
>> Your first example seems like the kind of code someone would
>> reasonably write. But your second and third examples are so bizarre
>> that making them errors makes a lot of sense.
>>
>> Exactly what type `cb` in the second example should even be is
>> unclear. Putting lambdas in function declarations is pretty dubious,
>> and even moreso for template functions that generate a new function
>> (and presumably a new type) for different instantiations. If you had
>> other template parameters there, you could start getting weird
>> behavior.
>>
>> And your last example is a perfect reason *why* it should fail to
>> compile. The person who wrote that function clearly thinks that the
>> `auto` here ought to work like `auto` when declaring a variable. And
>> it doesn't. They do not intend for the function to be a template, but
>> that's what they wrote. And they need to *know* that they wrote their
>> code incorrectly.
>>
>> We should not play into peoples' misconceptions about how language
>> features work.
>>
>>> Yes, there are solutions to make this work already today without any
>>> changes to the standard.
>>> Therefore the only motivation that I can give is, that this would make
>>> programming more convenient.
>>> In particular, together with abbreviated function templates.
>>>
>>> I would very much welcome your feedback!
>>> Thanks and kind regards,
>>> Kilian
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>
>