vc新建XML：并验证一个XML文档必须是一棵树而不是两棵树或者多棵树

 CComQIPtr<IXMLDOMDocument> spDomDoc;
 HRESULT hr;

 hr = spDomDoc.CoCreateInstance(CLSID_DOMDocument);
 if (FAILED(hr))
 {
  return;
 }

 
 CComQIPtr<IXMLDOMElement> spAppleElment;
 hr = spDomDoc->createElement(CComBSTR(L"FRUIT"), &spAppleElment);

 CComQIPtr<IXMLDOMNode> spDomDocNote(spDomDoc);
 CComQIPtr<IXMLDOMNode> spAppleElmentOut;
 spDomDocNote->appendChild(spAppleElment, &spAppleElmentOut);

 CComQIPtr<IXMLDOMNode> spAppleElmentOutTest;
 spDomDocNote->appendChild(spAppleElment, &spAppleElmentOutTest);//这里spAppleElmentOutTest肯定为NULL,因为一个XML文档是一棵树，而不是两棵树或者多棵树