Why is shared_ptr returning an empty list?

Question

I am working with Xerces-c (version 3.2) and I have this function that wraps around the XMLString::transcode method.

XMLCh* class_name::transcode(const std::string text) {
  auto returned = std::shared_ptr<XMLCh>(XMLString::transcode(text.c_str()), class_name::releaseXMLCh);
  return returned.get();
}

(XMLCh is a char16_t)

The point of this is to have a shared pointer that will call releaseXMLCh() when it is no longer needed. Here is the implementation of releaseXMLCh():

void class_name::releaseXMLCh(XMLCh* text) {
  XMLString::release(&text);
}

According to the Xerces-c documentation, XMLString::release needs to be called after calling XMLString::transcode to avoid a memory leak.

However, this function only ever returns "", regardless of what the input string is. I suspect that I am doing something wrong with the shared_ptr, as calling XML::transcode on its own works perfectly fine.

Does anyone know why this is happening?

Answer 1

Think about it:

• you create a local std::shared_ptr.
• You get the (raw) pointer to the internal object.
• The local std::shared_ptr goes out of scope and is destructed.

How many std::shared_ptrs will there be pointing to the internal object?

None. So it's also destructed. Thus, this will not work this way.

You are returning a pointer to a destroyed object, which causes Undefined Behavior.

You need a persisting smart pointer object (could likely even be a unique_ptr) or just use the old-fashioned new-delete.