QGIS » QGIS Application

In file qgsfeature.cpp, member function:
QString const& QgsFeature::wellKnownText() const
returns const reference to QString but in the body of
this function QString::null is returned.
null is an object of type QString::Null, so this return
expression returns reference to local (automatic) variable.
After program execution flow exits this function, this
reference is invalid and undefined behaviour can be
expected.

Here is small example that presents how easily the
problem explained above can cause serious crash:

#include <QCoreApplication>
#include <QString>
#include <iostream>

struct B
{
   QString const& foo()
   {
      return QString::null;
   }
};

int main(int argc, char *argv[])
{
   QCoreApplication app(argc, argv);
   B b;
   std::cout << b.foo().toStdString() << std::endl;
   //------------^^^^^^^^^^^^^^^^^^^ ka-Boom!
   return 0;
}

here, toStdString() is called on invalid reference to not existing object, so program will crash with "Access violation" error.

IMPORTANT!
I'd recommend to make a detailed review of all places
where const reference to QString is returned from
function or is passed to function and QString::null is
used as a default argument value.

This example above presents only return value case but
following example may also cause problems:

void foo(QString const& s = QString::null)
{
   // do something with s without checking
   // if its value is QString::null
   std::cout << s.toStdString() << std::endl;
   //-----------^^^^^^^^^^^^^^^ crash
}