[xml] more an the "ampersand problem"

[oliverst online de]

OK, I tried something different and here the code of what I tried:

        {
                const char* str = "<>\"'&";

                xmlNodePtr node = xmlNewNode(NULL, BAD_CAST "test");
                if( node ) {            
                        xmlNodeSetContent(node, BAD_CAST str);
                }

                string xml_str;
                CbXmlNodeToXmlFormatString(node, xml_str);
                cout << xml_str << endl << endl;        

                xmlNodePtr node2 = xmlNewNode(NULL, BAD_CAST str);

                CbXmlNodeToXmlFormatString(node2, xml_str);
                cout << xml_str << endl << endl;

                xmlNodePtr node3 = xmlNewNode(NULL, BAD_CAST "test");
                if( node3 ) {
                        xmlNewProp(node3, BAD_CAST str, BAD_CAST "test");
                }

                CbXmlNodeToXmlFormatString(node3, xml_str);
                cout << xml_str << endl << endl;

                xmlNodePtr node4 = xmlNewNode(NULL, BAD_CAST "test");
                if( node4 ) {
                        xmlNewProp(node4, BAD_CAST "test", BAD_CAST str);
                }

                CbXmlNodeToXmlFormatString(node4, xml_str);
                cout << xml_str << endl << endl;
        }

        {
                const char* str = "&lt;&gt;&quot;'&amp;";

                xmlNodePtr node = xmlNewNode(NULL, BAD_CAST "test");
                if( node ) {            
                        xmlNodeSetContent(node, BAD_CAST str);
                }

                string xml_str;
                CbXmlNodeToXmlFormatString(node, xml_str);
                cout << xml_str << endl << endl;        

                xmlNodePtr node2 = xmlNewNode(NULL, BAD_CAST str);

                CbXmlNodeToXmlFormatString(node2, xml_str);
                cout << xml_str << endl << endl;

                xmlNodePtr node3 = xmlNewNode(NULL, BAD_CAST "test");
                if( node3 ) {
                        xmlNewProp(node3, BAD_CAST str, BAD_CAST "test");
                }

                CbXmlNodeToXmlFormatString(node3, xml_str);
                cout << xml_str << endl << endl;

                xmlNodePtr node4 = xmlNewNode(NULL, BAD_CAST "test");
                if( node4 ) {
                        xmlNewProp(node4, BAD_CAST "test", BAD_CAST str);
                }

                CbXmlNodeToXmlFormatString(node4, xml_str);
                cout << xml_str << endl << endl;

        }

and the results:

error : unterminated entity reference
<test>&lt;&gt;"'</test>

<<>"'&/>

<test <>"'&="test"/>

<test test="&lt;&gt;&quot;'&amp;"/>

<test>&lt;&gt;"'&amp;</test>

<&lt;&gt;&quot;'&amp;/>

<test &lt;&gt;&quot;'&amp;="test"/>

<test test="&amp;lt;&amp;gt;&amp;quot;'&amp;amp;"/>


In case 1 it does not encode the & and does drop it with an error message, but it does encode all the others. 
Igor's explanation was totally acceptable to me, but as seen in case 4 everything is converted there.
The cases 2 and 3 can be ignored, as it's not a valid XML in both cases and I have to assure, that I use 
valid chars for the attribute or node name.
But in the last case I give the attribute an already properly encoded string and it does double-encode it. 
This also feels wrong to me, because you have to encode it, but it's nowhere mentioned, if libxml2 is doing 
the encoding for you or not. At least something like this should be added to the documentation.
If the first case would behave properly on the ampersand I would only have to care, that my input is proper 
UTF-8 and not about those "bad" special chars and would not have to convert anything by myself. Now I have to 
do the same as the libxml with my input string, parse it for the "unterminated entity" (the single-standing 
smapersand) and convert it.