[C++] Better support for building UnionArrays

[asfimport]

UnionBuilders (for both sparse and dense mode unions) are not currently supported by MakeBuilder or ArrayFromJSON. This increases friction when working with and testing against union arrays, and support should be added to both. For ArrayFromJSON each entry must be specified with a (type code, value) pair:

ArrayFromJSON(union_({field("lint", list(int32())), field("str", utf8())}), R"([
  [0, null],
  [1, "hello"],
  [0, [1, 2]],
  [1, "world"]
])");

DenseUnionBuilder currently requires the user to explicitly input offsets, but if it were modified to hold pointers to child builders (as ListBuilder, for example) then those offsets could be derived from the lengths of child builders (which is much more user friendly).

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++] add support for UnionArrays to Take and Filter (blocks)

PRs and other links:

• GitHub Pull Request #4781

_{Note: This issue was originally created as ARROW-5588. Please see the migration documentation for further details.}

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
Issue resolved by pull request 4781
#4781