二维vector 如何赋值？

#include<iostream>
#include<vector>
using namespace std;

int main(){
  vector<vector<int> > vec(4,vector<int>(5));
  int br[4][5] = {{1,2,8,9,5},{2,4,9,12,5},{4,7,10,13,5},{6,8,11,15,5}};
  for(int i = 0;i < 4;i++)
  {
    for(int j = 0;j < 4;j++)
    {
      vec[i][j] = br[i][j];
    }
  }
  cout<<vec.size()<<endl;//4
  
  cout<<vec[0].size();//5
  
}