[C++]: add version descriptor methods to group codecs.

Author: tmontgomery

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -362,12 +362,24 @@ private static CharSequence generateGroupProperty(
             indent + "    {\n" +
             indent + "        m_%2$s.wrapForEncode(m_buffer, count, m_positionPtr, m_actingVersion, m_bufferLength);\n" +
             indent + "        return m_%2$s;\n" +
-            indent + "    }\n",
+            indent + "    }\n\n",
             className,
             propertyName,
             cppTypeForNumInGroup
         ));
 
+        sb.append(String.format(
+            indent + "    static SBE_CONSTEXPR const std::uint64_t %1$sSinceVersion(void)\n" +
+            indent + "    {\n" +
+            indent + "         return %2$d;\n" +
+            indent + "    }\n\n" +
+            indent + "    bool %1$sInActingVersion(void)\n" +
+            indent + "    {\n" +
+            indent + "        return m_actingVersion >= %1$sSinceVersion();\n" +
+            indent + "    }\n",
+            propertyName,
+            (long)token.version()));
+
         return sb;
     }