How to declare a vector type

I am working on creating a board game.

For creating a grid of rows and columns, I used vectors and my code is as follows.

Grid(unsigned rows, unsigned cols) {
   //constructor code
    vector<vector<things*>>grid(nrows, vector< vector<people*>>(ncols));
}
~Grid() {
    //destructor code.
    vector<vector<things*>>().swap(grid);
}

But my destructor code is showing error in ().swap(grid).

Can anyone please help me.