No leaks from rapidxml

Today I was trying to find memory leaks from my project then I came across the below sample code

std::string VersionValue("1.0");
std::string EncodingValue("UTF-8");

rapidxml::xml_document<> XMLDoc;

rapidxml::xml_node<> * pHeaderNode = XMLDoc.allocate_node(rapidxml::node_declaration);
pHeaderNode->append_attribute(XMLDoc.allocate_attribute("version", VersionValue.c_str()));
pHeaderNode->append_attribute(XMLDoc.allocate_attribute("encoding", EncodingValue.c_str()));

I opened rapidxml code, inside allocate_attribute() I saw its allocating memory

xml_attribute<Ch> *attribute = new(memory) xml_attribute<Ch>;

and inside append_attribute() its assigning the memory to its member variable. There is no destructor function declared for xml_document. Then how its deleting the attributes? valgrind is returned 0 memory leaks from the above sample code. How its possible?

Solution

As mentioned in comments, placement new is the answer.

This behaviour is one of the major benefits - and one of the most common gotchas - with rapidxml: It doesn't make copies of any of the data passed as parameters.

For example, if your code looked like this:-

rapidxml::xml_document<> XMLDoc;

rapidxml::xml_node<> * pHeaderNode = 
XMLDoc.allocate_node(rapidxml::node_declaration);

{
  std::string VersionValue("1.0");
  std::string EncodingValue("UTF-8");
  pHeaderNode->append_attribute(XMLDoc.allocate_attribute("version", VersionValue.c_str()));
  pHeaderNode->append_attribute(XMLDoc.allocate_attribute("encoding", EncodingValue.c_str()))
}

... then you'll usually have major problems because the attribute strings will go out of scope but rapidxml will continue to hold stale pointers to them.