ARROW-3475: [C++] Allow builders to finish to the corresponding array type

[bkietz]

Allows Int64Builder to finish into shared_ptr<Int64Array>, ListBuilder to finish into shared_ptr<ListArray>, etc.

[bkietz]

Hmmm, I'm not sure how to resolve this doc error. Doxygen produces an extra declaration of ArrayBuilder::Finish from the explicit using. Without the explicit using, the base class' overload for plain Array doesn't participate in overload resolution.

I could rename the method to FinishTyped but that seems like a sloppy solution. Is there a doxygen tag/option I could use to explicitly ignore the declaration from using?

[wesm]

One option is to Call this Finish simply and add a DCHECK to assert that the runtime type matches ArrayType. I tried this here:

wesm@0be7cda

I'm not sure what pitfalls there might be from that

[bkietz]

That's an option. I avoided it because it seems sloppy/user-hostile to allow something to compile which could easily be caught as an error:

Int64Builder i64_b;
// ...
std::shared_ptr<Int32Array> i32_a;
i64_b.Finish(&i32_a); // would compile, but fail at runtime

[bkietz]

Another way would be to make this a non-member method:

Int64Builder builder;
// ...
std::shared_ptr<Int64Array> array;
Finish(&builder, &array); // type match guaranteed at compile time

[wesm]

Gotcha. Well, I think it's OK to accept things as they are now, the boilerplate does not seem too onerous

[wesm]

+1