Question

Let’s we have

std::vector <std::vector <unsigned short int>> face;
face.resize(nElm);

Its OK to resize() for the first dimension. However, I also want to reserve() memory for the elements of face; I mean for the second dimension. (I am aware of the difference between resize() and reserve())

Answer 1

Just do

face.resize(nElm);
for(auto &i : face) i.resize(nDim2);

or if you do not use c++11:

face.resize(nElm);
for(std::vector < std::vector < unsigned short int> >::iterator it
                =face.begin();it!=face.end();++it) {
   it->resize(dim2);
}

If you want to just reserve for the second dimension then just do that instead of resize