11.01.2017, 21:55, "Geoffrey Garen" <[email protected]>:
> I’m open to auto, but skeptical.
>
> (1) Research-ability. I read a lot of code that is new to me, that I have
> never written. I find type statements to be useful as documentation for where
> to look for more information about how data structures and algorithms relate
> to each other. Traversing a tree or list of data structures by type name
> while reading code is a common task for me. But I can’t select and search for
> auto, auto*, or auto&.
>
> (2) Mechanics. It’s totally true that if you loop over one collection and put
> the values into another collection, subject to some filter, the types
> involved aren’t really interesting to reviewing the filter. But if you want
> to review low-level mechanics like object lifetime and thread safety,
> suddenly the types are really important. I think the “I changed RefPtr to
> auto and then crashed” examples in this thread are about mechanics.
>
> I find it strange that, as JavaScript authors adopt tools like TypeScript
> because they find type-less programming to be unmaintainable, C++ authors
> move toward type-less programming. Perhaps the grass is always greener on the
> other side. Or perhaps we can strike a balance and put types where they help
> us.
>
> Two arguments made in favor of auto don’t fully convince me:
>
> (1) You were always able to hide the types of certain expressions.
>
> I’m hoping the point of this argument is not to say “in for a penny, in for a
> pound”, but rather to raise our awareness that we’ve been OK with certain
> type-less expressions all along. I agree: I'm OK with hiding types in some
> places, especially if surrounding code already provides some kind of type
> foothold.
Please don't confuse dynamic typing and static typing with type deduction.
>
> At the same time, I sometimes break expressions into statements to improve
> clarity and research-ability or to achieve certain mechanics.
>
> Maybe this answers Darin’s question:
>
>> Here’s one thing to consider: Why is it important to see the type of
>> foundSource->value, but not important to see the type of
>> shifts.take(source)? In both cases they need to be Vector<ShufflePair>.
>
> In the cited code, ‘shifts’ is declared as HashMap<Arg, Vector<ShufflePair>>,
> and HashMap ’take' is well understood to return its value type, so no further
> comment about type is necessary for me on this line.
>
> Following this logic, I might be OK with eliding the type of
> foundSource->value. But we’re starting to reach a limit. Each time we do this
> we create an auto link in the type chain. If I have to trace back three autos
> deep in order to understand a type, I’m gonna have a bad time. Each type
> declaration is a stopping point for research. Therefore, I don’t prefer the
> fully auto version of this code.
>
> Following this logic, I would be OK with const auto& a little later in this
> algorithm:
>
>> bool isRotate = false;
>> for (const ShufflePair& pair : currentPairs) {
>> if (pair.dst() == originalSrc) {
>> isRotate = true;
>> break;
>> }
>> }
>
> Might as well use const auto& for ‘pair’: The type of currentPairs is
> well-documented locally. If currentPairs were a data member, and not declared
> in the same screenful of code, I would want to write out a type for ‘pair’.
> Otherwise, I would have to open another file to understand this line.
>
> But somebody said we don’t like const auto&? Is this a gotcha somehow?
>
> (2) You can’t be sure of the stated type, since a type conversion may have
> happened.
>
> Knowing that a value is equivalent to a certain type still provides a
> foothold for research and understanding mechanics.
>
> We only allow automatic type conversion when we consider types to be
> reasonably equivalent. We consider it bad style to convert willy-nilly, and
> we use the explicit keyword to avoid errant conversions. For example, we
> don’t allow RefPtr to automatically convert to raw pointer.
>
> Geoff
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--
Regards,
Konstantin
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