[C++] Skip the OTF var data length prefix check for var data absent at the acting version

sbe-tool/src/main/cpp/otf/OtfMessageDecoder.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2025 Real Logic Limited.
+ * Copyright 2013-2026 Real Logic Limited.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -290,7 +290,7 @@ std::size_t decodeData(
         Token &lengthToken = tokens->at(tokenIndex + 2);
         Token &dataToken = tokens->at(tokenIndex + 3);
 
-        if ((bufferIndex + dataToken.offset()) > length)
+        if (isPresent && (bufferIndex + dataToken.offset()) > length)
         {
             throw std::runtime_error("length too short for data length field");
         }

sbe-tool/src/test/cpp/CMakeLists.txt

@@ -114,6 +114,7 @@ sbe_test(FieldAccessOrderCheckTest codecs)
 target_compile_definitions(FieldAccessOrderCheckTest PRIVATE SBE_ENABLE_PRECEDENCE_CHECKS_IN_TESTS)
 sbe_test(VersionedMessageTest codecs)
 sbe_test(Uint64LengthTest codecs)
+sbe_test(OtfAbsentVarDataTest codecs)
 
 # OtfMessageEncoderTest lives in the otf/ subdirectory and provides its own main() that takes the
 # IR file path as argv[1]; it is handed the generated code-generation-schema IR.

sbe-tool/src/test/cpp/OtfAbsentVarDataTest.cpp

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2026 Real Logic Limited.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include <cstdint>
+#include <memory>
+#include <stdexcept>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "otf/IrDecoder.h"
+#include "otf/OtfMessageDecoder.h"
+
+using namespace sbe::otf;
+
+/*
+ * AddVarDataV1 in field-order-check-schema.xml is an int32 field followed by var data added in version 1, so a
+ * version 0 message is the 4 byte block alone.
+ */
+static const char *SCHEMA_FILENAME = "field-order-check-schema.sbeir";
+static const int ADD_VAR_DATA_V1_TEMPLATE_ID = 1019;
+static const std::uint64_t BLOCK_LENGTH = 4;
+
+class OtfAbsentVarDataTest : public testing::Test, public OtfMessageDecoder::BasicTokenListener
+{
+public:
+    void SetUp() override
+    {
+        ASSERT_GE(m_irDecoder.decode(SCHEMA_FILENAME), 0);
+        m_messageTokens = m_irDecoder.message(ADD_VAR_DATA_V1_TEMPLATE_ID);
+        ASSERT_TRUE(m_messageTokens != nullptr);
+    }
+
+    void onVarData(Token &, const char *, std::uint64_t length, Token &) override
+    {
+        m_varDataCount++;
+        m_varDataLength = length;
+    }
+
+    std::size_t decode(const std::vector<char> &buffer, const std::uint64_t actingVersion)
+    {
+        return OtfMessageDecoder::decode(
+            buffer.data(), buffer.size(), actingVersion, BLOCK_LENGTH, m_messageTokens, *this);
+    }
+
+protected:
+    IrDecoder m_irDecoder = {};
+    std::shared_ptr<std::vector<Token>> m_messageTokens;
+    int m_varDataCount = 0;
+    std::uint64_t m_varDataLength = 1;
+};
+
+TEST_F(OtfAbsentVarDataTest, shouldDecodeOlderVersionMessageEndingBeforeAbsentVarData)
+{
+    const std::vector<char> buffer = { 42, 0, 0, 0 };
+
+    EXPECT_EQ(decode(buffer, 0), buffer.size());
+    EXPECT_EQ(m_varDataCount, 1);
+    EXPECT_EQ(m_varDataLength, 0u);
+}
+
+TEST_F(OtfAbsentVarDataTest, shouldRejectMissingLengthOfPresentVarData)
+{
+    const std::vector<char> buffer = { 42, 0, 0, 0 };
+
+    EXPECT_THROW(decode(buffer, 1), std::runtime_error);
+}