[C++] Add ability to get a var data as a string_view. Issue aeron-io#550.

Author: mjpt777

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

@@ -489,6 +489,29 @@ private CharSequence generateVarData(final String className, final List<Token> t
  lengthByteOrderStr,
  lengthCppType));
 
+ sb.append(String.format("\n" +
+ indent + " #if __cplusplus >= 201703L\n" +
+ indent + " const std::string_view get%1$sAsStringView()\n" +
+ indent + " {\n" +
+ "%2$s" +
+ indent + " std::uint64_t lengthOfLengthField = %3$d;\n" +
+ indent + " std::uint64_t lengthPosition = sbePosition();\n" +
+ indent + " sbePosition(lengthPosition + lengthOfLengthField);\n" +
+ indent + " %5$s lengthFieldValue;\n" +
+ indent + " std::memcpy(&lengthFieldValue, m_buffer + lengthPosition, sizeof(%5$s));\n" +
+ indent + " std::uint64_t dataLength = %4$s(lengthFieldValue);\n" +
+ indent + " std::uint64_t pos = sbePosition();\n" +
+ indent + " const std::string_view result(m_buffer + pos, dataLength);\n" +
+ indent + " sbePosition(pos + dataLength);\n" +
+ indent + " return result;\n" +
+ indent + " }\n" +
+ indent + " #endif\n",
+ propertyName,
+ generateStringViewNotPresentCondition(token.version(), BASE_INDENT),
+ lengthOfLengthField,
+ lengthByteOrderStr,
+ lengthCppType));
+
  sb.append(String.format("\n" +
  indent + " %1$s &put%2$s(const std::string& str)\n" +
  indent + " {\n" +
@@ -844,6 +867,21 @@ private static CharSequence generateStringNotPresentCondition(final int sinceVer
  sinceVersion);
  }
 
+ private static CharSequence generateStringViewNotPresentCondition(final int sinceVersion, final String indent)
+ {
+ if (0 == sinceVersion)
+ {
+ return "";
+ }
+
+ return String.format(
+ indent + " if (m_actingVersion < %1$d)\n" +
+ indent + " {\n" +
+ indent + " return std::string_view(\"\");\n" +
+ indent + " }\n\n",
+ sinceVersion);
+ }
+
  private static CharSequence generateTypeFieldNotPresentCondition(final int sinceVersion, final String indent)
  {
  if (0 == sinceVersion)