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GH-49110: [C++] Add bounds checking to DataType::field() to return nullptr for out-of-bounds access #49111

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GH-49110: [C++] Add bounds checking to DataType::field() to return nullptr for out-of-bounds access #49111

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7 changes: 6 additions & 1 deletion cpp/src/arrow/type.h
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Original file line number Diff line number Diff line change
Expand Up @@ -150,7 +150,12 @@ class ARROW_EXPORT DataType : public std::enable_shared_from_this<DataType>,
bool Equals(const std::shared_ptr<DataType>& other, bool check_metadata = false) const;

/// \brief Return the child field at index i.
const std::shared_ptr<Field>& field(int i) const { return children_[i]; }
///
/// Returns a null shared_ptr if index is out of bounds [0, num_fields()).
const std::shared_ptr<Field>& field(int i) const {
static const std::shared_ptr<Field> kNullField;
return (i < 0 || i >= num_fields()) ? kNullField : children_[i];
}

/// \brief Return the children fields associated with this type.
const FieldVector& fields() const { return children_; }
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21 changes: 20 additions & 1 deletion cpp/src/arrow/type_test.cc
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Expand Up @@ -2029,8 +2029,27 @@ TEST(TestStructType, Basics) {
auto t3 = struct_({field("c", int8()), field("b", utf8())});
ASSERT_TRUE(t1->Equals(t2));
ASSERT_TRUE(!t1->Equals(t3));
}

// Test that out-of-bounds field access returns null
TEST(TestStructType, FieldAccessOutOfBounds) {
auto f0 = field("f0", int32());
auto f1 = field("f1", utf8());
auto f2 = field("f2", uint8());
StructType struct_type({f0, f1, f2});

ASSERT_EQ(struct_type.num_fields(), 3);

// Out-of-bounds indices return nullptr
ASSERT_EQ(struct_type.field(3), nullptr);
ASSERT_EQ(struct_type.field(4), nullptr);
ASSERT_EQ(struct_type.field(100), nullptr);
ASSERT_EQ(struct_type.field(-1), nullptr);

// TODO(wesm): out of bounds for field(...)
// All out-of-bounds accesses return the same static null
const auto& null1 = struct_type.field(3);
const auto& null2 = struct_type.field(100);
EXPECT_EQ(&null1, &null2);
}

TEST(TestStructType, GetFieldByName) {
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