Bug report #3920

Returning address of local variable

Added by Mateusz Loskot - over 17 years ago. Updated over 14 years ago.

Assignee:Gavin Macaulay -
Affected QGIS version: Regression?:No
Operating System:All Easy fix?:No
Pull Request or Patch supplied: Resolution:fixed
Crashes QGIS or corrupts data: Copied to github as #:13944


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

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.

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


#1 Updated by Gavin Macaulay - over 17 years ago

  • Status changed from Open to In Progress

Resolved the given example in 6a7570f4 (SVN r5229). Haven't looked elsewhere for the same problem.

QString is implicitly shared, but I'm not sure if that resolves the problem for local variables returned via references (it does resolve the problem for QStrings returned via a copy).

Leaving this open as a reminder to look for and correct other instances of the same problem

#2 Updated by Gavin Macaulay - over 17 years ago

  • Resolution set to fixed
  • Status changed from In Progress to Closed

The one in QgsFeature seems to be the only place where this is done, so I'm closing this ticket.

#3 Updated by Mateusz Loskot - over 17 years ago

Yes, I dis some searching for places with similar problem and I've not found any.
So, it seems to be the only such place.

#4 Updated by Anonymous over 14 years ago

Milestone Version 0.8 deleted

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