Debug-Mode different behaviour with string

[louis]

Hi,

i found an old code-snipplet in our project:

  QString version = "abcdefghi";
  const char* cVersion = version.toStdString().c_str();

  qDebug() << cVersion;
  qDebug() << version;

Output in normal run is:
abcdefghi
"abcdefghi"

Output in Debug-mode is:
??????????????????????????????????????????????????????????????5???f>
"abcdefghi"

Compiled both as Debug and tried Qt 5.1.1 and Qt 5.6.0 using GNU gdb 7.8 MinGw 4.9.2 32 bit.
Also the last part always differs a little. So it shall be a pointer-problem. But what bugs me is the different behaviour in running and debug-mode.

Is there a reason for this?

[andr]

This code is broken.

version.toStdString() creates a temporary std::string object, for which you use .c_str() to get a pointer to the internal storage. At the end of the full expression (essentially the ';' in that line) all temporaries from this full expression are destroyed. I.e. the .c_str() result now points to some released(!) memory. Any access to that invokes undefined behaviour, and you notice exactly that.

The correct way is to keep the temporary value alive:

  QString version = "abcdefghi";
  std::string sVersion = version.toStdString();
  const char* cVersion = sversion.c_str();

or, better, to be explicit about the encoding change that's implicit in the toStdString()

  QString version = "abcdefghi";
  QByteArray sVersion = version.toUtf8(); // or .toLatin1(), or .toLocal8Bit(), or..
  const char* cVersion = sversion.constData);

[louis]

Works! And i understand why ;) thank you!