use boost's blank instead of none_t

src/ifcparse/IfcWrite.cpp

@@ -83,7 +83,7 @@ IfcEntityList::ptr IfcWritableEntity::getInverse(IfcSchema::Type::Enum type, int
 std::string IfcWritableEntity::datatype() const { return IfcSchema::Type::ToString(_type); }
 Argument* IfcWritableEntity::getArgument (unsigned int i) {
 	if (args[i] == 0) {
-		_setArgument(i, boost::none);
+		_setArgument(i, boost::blank());
 	}
 	return args[i];
 }
@@ -301,7 +301,7 @@ void IfcWritableEntity::setArgument(int i,const std::vector< boost::dynamic_bits
 
 class SizeVisitor : public boost::static_visitor<int> {
 public:
-	int operator()(const boost::none_t& /*i*/) const { return -1; }
+	int operator()(const boost::blank& /*i*/) const { return -1; }
 	int operator()(const IfcWriteArgument::Derived& /*i*/) const { return -1; }
 	int operator()(const int& /*i*/) const { return -1; }
 	int operator()(const bool& /*i*/) const { return -1; }
@@ -385,7 +385,7 @@ private:
 public:
 	StringBuilderVisitor(std::ostringstream& stream, bool upper = false) 
 		: data(stream), upper(upper) {}
-	void operator()(const boost::none_t& /*i*/) { data << "$"; }
+	void operator()(const boost::blank& /*i*/) { data << "$"; }
 	void operator()(const IfcWriteArgument::Derived& /*i*/) { data << "*"; }
 	void operator()(const int& i) { data << i; }
 	void operator()(const bool& i) { data << (i ? ".T." : ".F."); }

src/ifcparse/IfcWrite.h

@@ -59,7 +59,7 @@ namespace IfcWrite {
 	private:
 		boost::variant<
 			// A null argument, it will always serialize to $
-			boost::none_t,
+			boost::blank,
 			// A derived argument, it will always serialize to *
 			Derived,
 			// An integer argument, e.g. 123