Sorry,

maybe I am too stupid, but your code doesn't work for me. I take your code and just changed the name of the class:
Qt Code: 
  1. #ifndef CONFIGURATION_H
  2. #define CONFIGURATION_H
  3.  
  4. #include <QtCore>
  5. #include <QtSql>
  6.  
  7. class configuration
  8. {
  9.   public:
  10.     static configuration* instance()
  11.     {
  12.       static QMutex mutex;
  13.       if ( !m_instance )
  14.       {
  15.         mutex.lock();
  16.         if ( !m_instance )
  17.           m_instance = new configuration;
  18.         mutex.unlock();
  19.       }
  20.       return m_instance;
  21.     }
  22.  
  23.   private:
  24.   configuration(){};
  25.     configuration( const configuration& _instance ){};
  26.     static configuration* m_instance;
  27.  
  28. // [...] some other functions
  29.  
  30. };
  31.  
  32. configuration* configuration::m_instance = NULL;
  33.  
  34. #endif
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When compiling I receive the error (which I don't understand == don't know what to do/where the error could be):
.comp/sw_normal.o: In function `operator delete(void*, void*)':
/usr/include/qt4/QtCore/qatomic_i386.h:62: multiple definition of `configuration::m_instance'
.comp/qvortaro.o:/home/lykurg/Programmierung/qvortaro/src/qvortaro.cpp:194: first defined here
collect2: ld returned 1 exit status


And by the way must it in your example in the wiki not be
Qt Code: 
  1. Singleton* Singleton::m_Instance = 0;
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instead of
Qt Code: 
  1. Singleton* m_Instance = 0;
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Thanks,
Lykurg

P.s: if I comment the function (on bottom of the file) in qvortaro.cpp:194 out, then this error pointed to now last function on the file.