[arrow] branch master updated: GH-15292: [C++] Typeclass alias is missing in ExtensionArray (#15293)

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The following commit(s) were added to refs/heads/master by this push:
     new df2aa384ee GH-15292: [C++] Typeclass alias is missing in ExtensionArray (#15293)
df2aa384ee is described below

commit df2aa384eedc6dfd2a816f66e3ef8af1ecda3e8f
Author: Jin Shang <sh...@gmail.com>
AuthorDate: Thu Jan 12 21:26:02 2023 +0800

    GH-15292: [C++] Typeclass alias is missing in ExtensionArray (#15293)
    
    # Which issue does this PR close?
    
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    Then could you also rename the pull request title in the following format?
    
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    Closes #15292
    
    # Rationale for this change
    
    <!--
     Why are you proposing this change? If this is already explained clearly in the issue then this section is not needed.
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    All array classes have the `using TypeClass = xxx` type alias except for ExtensionArray.
    
    With the alias, it will be easier to write templated inline visitors such as the following code block:
    
    ```Cpp
    class ExampleVisitor {
     public:
      template <typename T>
      Status Visit(const T& arr) {
        if constexpr (arrow::is_number_type<typename T::TypeClass>::value) {
          ///
        } else {
          ///
        }
        return Status::OK();
      }
    };
    ```
    
    Current it fails to compile with message: `No type named 'TypeClass' in 'arrow::ExtensionArray'`.
    * Closes: #15292
    
    Authored-by: Jin Shang <sh...@gmail.com>
    Signed-off-by: Antoine Pitrou <an...@python.org>
---
 cpp/src/arrow/extension_type.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/cpp/src/arrow/extension_type.h b/cpp/src/arrow/extension_type.h
index badbdbd981..dd004118e8 100644
--- a/cpp/src/arrow/extension_type.h
+++ b/cpp/src/arrow/extension_type.h
@@ -103,6 +103,7 @@ class ARROW_EXPORT ExtensionType : public DataType {
 /// \brief Base array class for user-defined extension types
 class ARROW_EXPORT ExtensionArray : public Array {
  public:
+  using TypeClass = ExtensionType;
   /// \brief Construct an ExtensionArray from an ArrayData.
   ///
   /// The ArrayData must have the right ExtensionType.