C++ returning pointer from function. Values are lost

Question

I have the the following:

void Class1::method()
{
  QStringList* file_list;
  collect_file_paths(file_list);    //Sends pointer to the method below
}

void Class1::collect_file_paths(QStringList* file_list)
{
  //Gather file paths
  DirectorySearch ds;
  connect(&ds, SIGNAL(updateStatus(QString)), this, SLOT(onStatusUpdate(QString)));
  file_list = ds.get_file_names(_strPath);  //Returns a pointer of QStringList
}

QStringList* DirectorySearch::get_file_names(QString path)
{
  QStringList *file_names = new QStringList;
  traverse(path, file_names);
  compare_existing(file_names);
  return file_names;  //returning pointer address
}

What is happening is that the memory address returned from get_file_names() is lost/deleted when I leave the scope of Class1::collect_file_paths(). It was my understanding that QStringList *file_names = new QStringList; in the DirectorySearch class in now on the memory heap. So it should be left on the heap until I call delete i.e. it is never out of scope. However, as mentioned above, the address/values of the QStingList are lost when I return to Class1::method() from collect_file_paths(file_list).

Can some explain what is happening?

Thanks.

Answer 1

When you do file_list = ds.get_file_names(_strPath); //Returns a pointer of QStringList you are changing the value of the file_list (a 32/64b memory location) NOT the value of the data that memory location represents.

You need to pass a reference to your pointer (or a pointer to a pointer)

void Class1::method()
{
  QStringList* file_list;
  collect_file_paths(&file_list);    //Sends pointer to pointer to the method below
}

void Class1::collect_file_paths(QStringList** file_list)
{
  //Gather file paths
  DirectorySearch ds;
  connect(&ds, SIGNAL(updateStatus(QString)), this, SLOT(onStatusUpdate(QString)));
  *file_list = ds.get_file_names(_strPath); //set value of pointer in caller to value returned by below function
}
...

or so