[jira] [Assigned] (ARROW-6326) [C++] Nullable fields when converting std::tuple to Table

["Uwe L. Korn (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-6326?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Uwe L. Korn reassigned ARROW-6326:
----------------------------------

    Assignee: Omer Ozarslan

> [C++] Nullable fields when converting std::tuple to Table
> ---------------------------------------------------------
>
>                 Key: ARROW-6326
>                 URL: https://issues.apache.org/jira/browse/ARROW-6326
>             Project: Apache Arrow
>          Issue Type: New Feature
>          Components: C++
>            Reporter: Omer Ozarslan
>            Assignee: Omer Ozarslan
>            Priority: Major
>              Labels: pull-request-available
>          Time Spent: 3h 10m
>  Remaining Estimate: 0h
>
> {{std::optional}} isn't used for representing nullable fields in Arrow's current STL conversion API since it requires C++17. Also there are other ways to represent an optional field other than {{std::optional}} such as using pointers or external implementations of optional ({{boost::optional}}, {{type_safe::optional}} and alike). 
> Since it is hard to maintain so many different kinds of specializations, introducing an {{Optional}} concept covering these classes could solve this issue and allow implementing nullable fields consistently.
> So, the gist of proposed change will be something along the lines of:
> {code:cpp}
> template<typename T>
> constexpr bool is_optional_like_v = ...;
> template<typename Optional>
> struct CTypeTraits<Optional, enable_if_t<is_optional_like_v<Optional>>> {
>    //...
> }
> template<typename Optional>
> struct ConversionTraits<Optional, enable_if_t<is_optional_like_v<Optional>>> : public CTypeTraits<Optional> {
>    //...
> }
> {code}
> For a type {{T}} to be considered as an {{Optional}}:
> 1) It should be convertible (implicitly or explicitly)  to {{bool}}, i.e. it implements {{[explicit] operator bool()}},
> 2) It should be dereferencable, i.e. it implements {{operator*()}}.
> These two requirements provide a generalized way of templating nullable fields based on pointers, {{std::optional}}, {{boost::optional}} etc. However, it would be better (necessary?) if this implementation should act as a default while not breaking existing specializations of users (e.g. an existing  implementation in which {{std::optional}} is specialized by user).
> Is there any issues this approach may cause that I may have missed?
> I will open a draft PR for working on that meanwhile.



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