Convert c++ vector to c-style pointer

Question

vector<vector<double> >a(3,vector<double>(4));
double *p = a[0];

Why this is wrong, a[0] is not the address of the first dimension of a?

Answer 1

Look here

vector<vector<double> >a(3,vector<double>(4));

You defined a as a vector having 3 elements of type vector<double>. So a[0] has type vector<double>. vector is a user defined type. It is not a pointer.