Qt: iterator of a vector in a QMap

Question

I am working with Qt and OpenCV and I would like to create an iterator with std::vector <cv::Rect_<int>> to have access to all cv::Rect_.
This vector is part of a QMap < int, std::vector <cv::Rect_<int>> > _facesframe;

So this is how I am trying to have access to these vectors:

                foreach (unsigned int frame , _imageGItem->_faceSampler._facesframe.keys() )
                {
                    std::vector <cv::Rect_<int>>::const_iterator it = _imageGItem->_faceSampler._facesframe.value(frame).begin();
                    if( it != _imageGItem->_faceSampler._facesframe.value(frame).end())
                    {
                        qDebug()<<"here";
                    }

                }

But the program crashes at the line if... because of an incompatible iterator.

Does someone know how to reach all cv::Rect_<int> of a QMap < int, std::vector <cv::Rect_<int>> > please?

Answer 1

This is because you are comparing iterators to different vectors.

const T QMap::value(const Key & key, const T & defaultValue = T()) const

Vector is returned by value, so this is copied.

You should use

T & QMap::operator[](const Key & key)

to correct this:

foreach (unsigned int frame , _imageGItem->_faceSampler._facesframe.keys() )
  {
    std::vector <cv::Rect_<int>>::const_iterator it =
                       _imageGItem->_faceSampler._facesframe[frame].begin();
    if( it != _imageGItem->_faceSampler._facesframe[frame].end())
      {
        qDebug()<<"here";
      }

  }

or (less efficient bacause of 1 copy being made):

std::vector <cv::Rect_<int>> v =   // this will copy
                            _imageGItem->_faceSampler._facesframe.value(frame);
std::vector <cv::Rect_<int>>::const_iterator it = v.begin();
if( it != v.end())
  {
    qDebug()<<"here";
  }

Answer 2

QMap::value returns by value. Which means it's returning a copy of the vector, and of course your iterator points to a different vector (a different copy). Change it to use the non-const version operator[] instead (as the const version of that also returns by value). Or just use std::map, which offers a much better interface in this regard.