From cf259284b9cebda9ce8475b0090e46b0f8d3fa9c Mon Sep 17 00:00:00 2001
From: Philippe Verney <philippe.verney@f2i-consulting.com>
Date: Wed, 12 Nov 2025 16:14:12 +0100
Subject: [PATCH 1/2] C++ std::optional Specific Encoder and Decoder

---
 lang/c++/include/avro/Specific.hh | 39 +++++++++++++++++++++++++++++++
 lang/c++/test/SpecificTests.cc    | 15 ++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/lang/c++/include/avro/Specific.hh b/lang/c++/include/avro/Specific.hh
index fc28b3f5e4b..cfbc07c9a23 100644
--- a/lang/c++/include/avro/Specific.hh
+++ b/lang/c++/include/avro/Specific.hh
@@ -22,6 +22,7 @@
 #include "array"
 #include <algorithm>
 #include <map>
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -165,6 +166,44 @@ struct codec_traits<double> {
     }
 };
 
+/**
+* codec_traits for Avro optional.
+*/
+template<typename T>
+struct codec_traits<std::optional<T>> {
+    /**
+	* Encodes a given value.
+	*/
+    static void encode(Encoder &e, const std::optional<T> &b) {
+        if (b) {
+            e.encodeUnionIndex(1);
+            avro::encode(e, b.value());
+        } else {
+            e.encodeUnionIndex(0);
+            e.encodeNull();
+        }
+    }
+
+    /**
+	* Decodes into a given value.
+	*/
+    static void decode(Decoder &d, std::optional<T> &s) {
+        size_t n = d.decodeUnionIndex();
+        if (n >= 2) { throw avro::Exception("Union index too big"); }
+        switch (n) {
+            case 0: {
+                d.decodeNull();
+                s = std::nullopt;
+            } break;
+            case 1: {
+                T t;
+                avro::decode(d, t);
+                s.emplace(t);
+            } break;
+        }
+    }
+};
+
 /**
  * codec_traits for Avro string.
  */
diff --git a/lang/c++/test/SpecificTests.cc b/lang/c++/test/SpecificTests.cc
index 72f2897e45b..91c5ce6d551 100644
--- a/lang/c++/test/SpecificTests.cc
+++ b/lang/c++/test/SpecificTests.cc
@@ -24,6 +24,7 @@
 
 using std::array;
 using std::map;
+using std::optional;
 using std::string;
 using std::unique_ptr;
 using std::vector;
@@ -127,6 +128,18 @@ void testDouble() {
     BOOST_CHECK_CLOSE(b, n, 0.00000001);
 }
 
+void testNonEmptyOptional() {
+    optional<int64_t> n = -109;
+    optional<int64_t> b = encodeAndDecode(n);
+    BOOST_CHECK_EQUAL(b.value(), n.value());
+}
+
+void testEmptyOptional() {
+    optional<int64_t> n;
+    optional<int64_t> b = encodeAndDecode(n);
+    BOOST_CHECK(!b.has_value());
+}
+
 void testString() {
     string n = "abc";
     string b = encodeAndDecode(n);
@@ -191,6 +204,8 @@ init_unit_test_suite(int /*argc*/, char * /*argv*/[]) {
     ts->add(BOOST_TEST_CASE(avro::specific::testLong));
     ts->add(BOOST_TEST_CASE(avro::specific::testFloat));
     ts->add(BOOST_TEST_CASE(avro::specific::testDouble));
+    ts->add(BOOST_TEST_CASE(avro::specific::testNonEmptyOptional));
+    ts->add(BOOST_TEST_CASE(avro::specific::testEmptyOptional));
     ts->add(BOOST_TEST_CASE(avro::specific::testString));
     ts->add(BOOST_TEST_CASE(avro::specific::testBytes));
     ts->add(BOOST_TEST_CASE(avro::specific::testFixed));

From e7022101d22c8d32abfaea5d4f3e59478d5fb476 Mon Sep 17 00:00:00 2001
From: Philippe Verney <philippe.verney@f2i-consulting.com>
Date: Thu, 13 Nov 2025 10:19:37 +0100
Subject: [PATCH 2/2] In place construction for optional

Add missing header.
Improve error message.
Document expected corresponding schema.
Prefere optional.reset() over optional = std::nullopt
---
 lang/c++/include/avro/Specific.hh | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/lang/c++/include/avro/Specific.hh b/lang/c++/include/avro/Specific.hh
index cfbc07c9a23..edd2c92d867 100644
--- a/lang/c++/include/avro/Specific.hh
+++ b/lang/c++/include/avro/Specific.hh
@@ -30,6 +30,7 @@
 #include "Config.hh"
 #include "Decoder.hh"
 #include "Encoder.hh"
+#include "Exception.hh"
 
 /**
  * A bunch of templates and specializations for encoding and decoding
@@ -167,7 +168,7 @@ struct codec_traits<double> {
 };
 
 /**
-* codec_traits for Avro optional.
+* codec_traits for Avro optional assumming that the schema is ["null", T].
 */
 template<typename T>
 struct codec_traits<std::optional<T>> {
@@ -189,16 +190,15 @@ struct codec_traits<std::optional<T>> {
 	*/
     static void decode(Decoder &d, std::optional<T> &s) {
         size_t n = d.decodeUnionIndex();
-        if (n >= 2) { throw avro::Exception("Union index too big"); }
+        if (n >= 2) { throw avro::Exception("Union index too big for optional (expected 0 or 1, got " + std::to_string(n) + ")"); }
         switch (n) {
             case 0: {
                 d.decodeNull();
-                s = std::nullopt;
+                s.reset();
             } break;
             case 1: {
-                T t;
-                avro::decode(d, t);
-                s.emplace(t);
+                s.emplace();
+                avro::decode(d, *s);
             } break;
         }
     }