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reserve() does NOT allocate any data. the correct way to do this would be:

// a row vector, safe to iterate from pb[0] to pb[9], given:   int *pB = B.ptr<int>(0);
Mat_<int> B(1,10); 

// a col vector, safe to do:  int *pB = B.ptr<int>(i); and pb[0]
Mat_<int> B(10,1);

Mat is essentially a 2d container, if you want it to behave like a std::vector, why not use one in the 1st place ?

reserve() does NOT allocate any data. the correct way to do this would be:

// a row vector, 
Mat_<int> B(1,10); 
int *pB = B.ptr<int>(0);
// now it's safe to iterate from pb[0] to pb[9], given: pb[9]   int *pB = B.ptr<int>(0);
Mat_<int> B(1,10); 

// a col vector, safe to do:  vector
Mat_<int> B(10,1); 
int *pB = B.ptr<int>(i); and pb[0]
Mat_<int> B(10,1);
B.ptr<int>(i);
// careful, all you can use is pb[0] !

Mat is essentially a 2d container, if you want it to behave like a std::vector, why not use one in the 1st place ?