Well, it's fairly obvious that sensors goes out of scope when the initFunction() ends, so trying to use it in readTemp() doesn't make sense.
Your class needs a constructor. The constructor does some stuff. The initFunction() might be needed, or it might not. In any case, it is typical, in the Arduino world, to call such a method begin().
The constructor needs to create the oneWire and sensors instances, which have to be declared as private members of the class.
Your class header file would have:
private:
OneWire oneWire;
DallasTemperature sensors;
public:
MyLib();
added.
Your source file would have:
MyLib::MyLib(): oneWire(3), sensors(&oneWire)
{
// Anything else...
}
added.