From 0678222faa86111eb3cc2e2ada341bf390231982 Mon Sep 17 00:00:00 2001
From: Naurder <48394023+Naurder@users.noreply.github.com>
Date: Wed, 10 Jun 2026 22:41:58 +0200
Subject: [PATCH 1/3] GH-48740: [C++] Add missing CTypeTraits for decimal types

---
 cpp/src/arrow/type_traits.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/cpp/src/arrow/type_traits.h b/cpp/src/arrow/type_traits.h
index 1b7a02e1085a..78fb08cd3058 100644
--- a/cpp/src/arrow/type_traits.h
+++ b/cpp/src/arrow/type_traits.h
@@ -103,6 +103,16 @@ struct CTypeTraits {};
 
 /// \addtogroup type-traits
 /// @{
+template <>
+struct CTypeTraits<Decimal128> {
+  using ArrowType = Decimal128Type;
+};
+
+template <>
+struct CTypeTraits<Decimal256> {
+  using ArrowType = Decimal256Type;
+};
+
 template <>
 struct TypeTraits<NullType> {
   using ArrayType = NullArray;

From c1e09556ff41fe7c33ead3135a0a14076134582a Mon Sep 17 00:00:00 2001
From: Naurder <48394023+Naurder@users.noreply.github.com>
Date: Thu, 11 Jun 2026 19:15:50 +0200
Subject: [PATCH 2/3] apply reviewer suggestion: inherit from TypeTraits

---
 cpp/src/arrow/type_traits.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/cpp/src/arrow/type_traits.h b/cpp/src/arrow/type_traits.h
index 78fb08cd3058..900c3e05a56b 100644
--- a/cpp/src/arrow/type_traits.h
+++ b/cpp/src/arrow/type_traits.h
@@ -104,12 +104,12 @@ struct CTypeTraits {};
 /// \addtogroup type-traits
 /// @{
 template <>
-struct CTypeTraits<Decimal128> {
+struct CTypeTraits<Decimal128> : TypeTraits<Decimal128Type> {
   using ArrowType = Decimal128Type;
 };
 
 template <>
-struct CTypeTraits<Decimal256> {
+struct CTypeTraits<Decimal256> : TypeTraits<Decimal256Type> {
   using ArrowType = Decimal256Type;
 };
 

From 220a94b8959e369d73ae34b7726b8041dd4b168f Mon Sep 17 00:00:00 2001
From: Naurder <48394023+Naurder@users.noreply.github.com>
Date: Fri, 12 Jun 2026 18:23:04 +0200
Subject: [PATCH 3/3] Address review comments: move CTypeTraits, add Dec32/64
 and tests

---
 cpp/src/arrow/type_test.cc  | 14 ++++++++++++++
 cpp/src/arrow/type_traits.h | 30 ++++++++++++++++++++----------
 2 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/cpp/src/arrow/type_test.cc b/cpp/src/arrow/type_test.cc
index 6197ad58eb40..133afbfa0c58 100644
--- a/cpp/src/arrow/type_test.cc
+++ b/cpp/src/arrow/type_test.cc
@@ -1493,6 +1493,20 @@ PRIMITIVE_TEST(DoubleType, double, DOUBLE, "double");
 
 PRIMITIVE_TEST(BooleanType, bool, BOOL, "bool");
 
+TEST(TypesTest, DecimalTraits) {
+  static_assert(std::is_same_v<TypeTraits<Decimal32Type>::CType, Decimal32>);
+  static_assert(std::is_same_v<CTypeTraits<Decimal32>::ArrowType, Decimal32Type>);
+
+  static_assert(std::is_same_v<TypeTraits<Decimal64Type>::CType, Decimal64>);
+  static_assert(std::is_same_v<CTypeTraits<Decimal64>::ArrowType, Decimal64Type>);
+
+  static_assert(std::is_same_v<TypeTraits<Decimal128Type>::CType, Decimal128>);
+  static_assert(std::is_same_v<CTypeTraits<Decimal128>::ArrowType, Decimal128Type>);
+
+  static_assert(std::is_same_v<TypeTraits<Decimal256Type>::CType, Decimal256>);
+  static_assert(std::is_same_v<CTypeTraits<Decimal256>::ArrowType, Decimal256Type>);
+}
+
 TEST(TestBinaryType, ToString) {
   BinaryType t1;
   BinaryType e1;
diff --git a/cpp/src/arrow/type_traits.h b/cpp/src/arrow/type_traits.h
index 900c3e05a56b..cbf2fcd1a22e 100644
--- a/cpp/src/arrow/type_traits.h
+++ b/cpp/src/arrow/type_traits.h
@@ -103,16 +103,6 @@ struct CTypeTraits {};
 
 /// \addtogroup type-traits
 /// @{
-template <>
-struct CTypeTraits<Decimal128> : TypeTraits<Decimal128Type> {
-  using ArrowType = Decimal128Type;
-};
-
-template <>
-struct CTypeTraits<Decimal256> : TypeTraits<Decimal256Type> {
-  using ArrowType = Decimal256Type;
-};
-
 template <>
 struct TypeTraits<NullType> {
   using ArrayType = NullArray;
@@ -367,6 +357,26 @@ struct TypeTraits<Decimal256Type> {
   constexpr static bool is_parameter_free = false;
 };
 
+template <>
+struct CTypeTraits<Decimal32> : TypeTraits<Decimal32Type> {
+  using ArrowType = Decimal32Type;
+};
+
+template <>
+struct CTypeTraits<Decimal64> : TypeTraits<Decimal64Type> {
+  using ArrowType = Decimal64Type;
+};
+
+template <>
+struct CTypeTraits<Decimal128> : TypeTraits<Decimal128Type> {
+  using ArrowType = Decimal128Type;
+};
+
+template <>
+struct CTypeTraits<Decimal256> : TypeTraits<Decimal256Type> {
+  using ArrowType = Decimal256Type;
+};
+
 template <>
 struct TypeTraits<BinaryType> {
   using ArrayType = BinaryArray;