[GitHub] [arrow-rs] tustvold commented on a diff in pull request #3117: Add downcast_array (#2901)

[GitBox <gi...@apache.org>]

tustvold commented on code in PR #3117:
URL: https://github.com/apache/arrow-rs/pull/3117#discussion_r1022291987


##########
arrow-array/src/cast.rs:
##########
@@ -550,6 +551,38 @@ array_downcast_fn!(as_union_array, UnionArray);
 array_downcast_fn!(as_map_array, MapArray);
 array_downcast_fn!(as_decimal_array, Decimal128Array);
 
+/// Downcasts a `dyn Array` to a concrete type
+///
+/// ```
+/// # use arrow_array::{BooleanArray, Int32Array, RecordBatch, StringArray};
+/// # use arrow_array::cast::downcast_array;
+/// struct ConcreteBatch {
+///     col1: Int32Array,
+///     col2: BooleanArray,
+///     col3: StringArray,
+/// }
+///
+/// impl ConcreteBatch {
+///     fn new(batch: &RecordBatch) -> Self {
+///         Self {
+///             col1: downcast_array(batch.column(0).as_ref()),
+///             col2: downcast_array(batch.column(1).as_ref()),
+///             col3: downcast_array(batch.column(2).as_ref()),
+///         }
+///     }
+/// }
+/// ```
+///
+/// # Panics
+///
+/// Panics if array is not of the correct data type
+pub fn downcast_array<T>(array: &dyn Array) -> T
+where
+    T: From<ArrayData>,
+{
+    T::from(array.data().clone())

Review Comment:
   There may be some mechanism to avoid this clone, if/when we work this out this can be optimised. In practice, this clone should be irrelevant, as it all the underlying buffers are reference counted



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