simgear/props/props_io.*: added class for inline calling of writeProperties().

E.g. allows:

    SG_LOG(SG_VIEW, SG_ALERT, "returning:\n" << writePropertiesInline(config, true /*write_all*/));

simgear/props/props_io.cxx

@@ -689,6 +689,24 @@ writeProperties (ostream &output, const SGPropertyNode * start_node,
   output << "</PropertyList>" << endl;
 }
 
+writePropertiesInline::writePropertiesInline(
+        const SGPropertyNode * start_node,
+        bool write_all,
+        SGPropertyNode::Attribute archive_flag
+        )
+:
+m_start_node(start_node),
+m_write_all(write_all),
+m_archive_flag(archive_flag)
+{
+}
+
+std::ostream& operator<< (std::ostream& out, const writePropertiesInline& wpm)
+{
+    writeProperties(out, wpm.m_start_node, wpm.m_write_all, wpm.m_archive_flag);
+    return out;
+}
+
 
 void
 writeProperties (const SGPath &path, const SGPropertyNode * start_node,

simgear/props/props_io.hxx

@@ -49,6 +49,23 @@ void writeProperties (std::ostream &output, const SGPropertyNode * start_node,
 		      bool write_all = false,
 		      SGPropertyNode::Attribute archive_flag = SGPropertyNode::ARCHIVE);
 
+/**
+ * Convenience manipulator for calling writeProperties(), e.g.:
+ * std::cerr << "node is:\n" << writePropertiesStream(node, true) << "\n";
+ */
+struct writePropertiesInline
+{
+    writePropertiesInline(
+            const SGPropertyNode * start_node,
+		    bool write_all = false,
+		    SGPropertyNode::Attribute archive_flag = SGPropertyNode::ARCHIVE
+            );
+    const SGPropertyNode * m_start_node;
+    bool m_write_all;
+    SGPropertyNode::Attribute m_archive_flag;
+};
+
+std::ostream& operator<< (std::ostream& out, const writePropertiesInline& wpm);
 
 /**
  * Write properties to an XML file.