[C++]: add wrapAndApplyHeader for ease of use.

Author: tmontgomery

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

@@ -1693,6 +1693,19 @@ private CharSequence generateMessageFlyweightCode(final String className, final
             "        reset(buffer, offset, bufferLength, sbeBlockLength(), sbeSchemaVersion());\n" +
             "        return *this;\n" +
             "    }\n\n" +
+            "    %10$s &wrapAndApplyHeader( " +
+            "char *buffer, const std::uint64_t offset, const std::uint64_t bufferLength)\n" +
+            "    {\n" +
+            "        MessageHeader hdr(buffer + offset, bufferLength, sbeSchemaVersion());\n\n" +
+            "        hdr\n" +
+            "            .blockLength(sbeBlockLength())\n" +
+            "            .templateId(sbeTemplateId())\n" +
+            "            .schemaId(sbeSchemaId())\n" +
+            "            .version(sbeSchemaVersion());\n\n" +
+            "        reset(buffer + offset + MessageHeader::encodedLength(), 0, bufferLength, sbeBlockLength(), " +
+            "sbeSchemaVersion());\n" +
+            "        return *this;\n" +
+            "    }\n\n" +
             "    %10$s &wrapForDecode(\n" +
             "         char *buffer, const std::uint64_t offset, const std::uint64_t actingBlockLength,\n" +
             "         const std::uint64_t actingVersion, const std::uint64_t bufferLength)\n" +