replace
Qt Code: 
  1. std::vector <double>::iterator It1;
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by
Qt Code: 
  1. typename std::vector<T>::iterator It1;
  2. // Or make it a const_iterator since you do not want to modify things.
  3. // For the same reason your method could (should) be declared const.
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(You might want to provide a typedef for that in your class.)

HTH