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

["Omer Ozarslan (Jira)" <ji...@apache.org>]

Omer Ozarslan created ARROW-6326:
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             Summary: [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


{{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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