C++ How does a multi-dimensional vector grow internally?

Question

Consider a two-dimensional vector with integers, but initially only with one element:

std::vector< std::vector <int> > vec( 1, std::vector<int>( 1, 0 ) );

Now I want to make the vector in both dimensions larger, so that the vector results always in a m x m matrix.

Will the following two commands do that:

vec.push_back( std::vector<int> );
vec[0].push_back( 0 );

or will just the first row and respectively the first column increase by an element?

Answer 1

Only the first row will enlarge. The fact the vectors are all grouped together doesn't make them enlarge together.

Answer 2

If you are creating a vector of vectors, you will need to increase every row if you want the width of every row to increase....