[C++] Extend isEmpty() method functionality of PR aeron-io#575 to C++.

Author: mjpt777

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

@@ -584,16 +584,25 @@ private void generateChoiceSet(final List<Token> tokens) throws IOException
  out.append(generateClassDeclaration(bitSetName));
  out.append(generateFixedFlyweightCode(bitSetName, tokens.get(0).encodedLength()));
 
- out.append(String.format("\n" +
+ out.append(String.format(
  " %1$s &clear()\n" +
  " {\n" +
  " %2$s zero = 0;\n" +
  " std::memcpy(m_buffer + m_offset, &zero, sizeof(%2$s));\n" +
  " return *this;\n" +
- " }\n\n",
+ " }\n",
  bitSetName,
  cppTypeName(tokens.get(0).encoding().primitiveType())));
 
+ out.append(String.format("\n" +
+ " bool isEmpty() const\n" +
+ " {\n" +
+ " %1$s val;\n" +
+ " std::memcpy(&val, m_buffer + m_offset, sizeof(%1$s));\n" +
+ " return 0 == val;\n" +
+ " }\n",
+ cppTypeName(tokens.get(0).encoding().primitiveType())));
+
  out.append(generateChoices(bitSetName, tokens.subList(1, tokens.size() - 1)));
  out.append("};\n");
  out.append(CppUtil.closingBraces(ir.namespaces().length)).append("#endif\n");