const float myValue = 1.2f; 
const float defaultValue = 0.1f; 

QSettings settings( QSettings::UserScope, "QtCompany", "PreferencesTest" ); 
settings.beginGroup( "/Preferences" ); 

settings.setValue( "/My value", myValue ); 
float myValueReadFromMemory = settings.value( "/My value", defaultValue ).toFloat(); 
settings.endGroup(); 

settings.sync(); 

QSettings settingsToRead( QSettings::UserScope, "QtCompany", "PreferencesTest" ); 
settingsToRead.beginGroup( "/Preferences" ); 

float myValueRead = settingsToRead.value( "/My value", defaultValue ).toFloat(); 

settingsToRead.endGroup(); 

printf( "value %f default %f read %f read(from memory) %f\n", 
myValue, defaultValue, myValueRead, myValueReadFromMemory ); 

On Mavericks: 
value 1.200000 default 0.100000 read 1.200000 read(from memory) 1.200000 

On Yosemite: 
value 1.200000 default 0.100000 read 0.000000 read(from memory) 0.000000