Use vector to write a matrix template

Vectors can write matrices over two dimensions. I was thinking about this problem when I was reading vector and string. How it should be written. Using a similar idea to a two-dimensional array, if the vector holds a vector that is not available, and the access operation is much more convenient than the array. The procedure is as follows:

#include <iostream>
#include <vector>
using namespace std;
Template<typename t>//Template
class Matrix {public
:
	matrix (int rows, int cols): arr (rows)
	{/
		* for (rsize_t t = 0;t < rows;t++)
			arr[t].resize (cols); */For
		(auto &thisrow:arr)//auto automatically inferred
			. Resize (cols);
	}
	Matrix (vector<vector<t>> V): arr{v}	//Copy copy constructor
	{}
	matrix (vector<vector<t> > &&v): arr{std::move (v)}//Mobile copy constructor
	{}
	const vector<t> &operator[] (int row) const// Overloaded subscript reference
	{return
		arr[row];
	}
	vector<t>& operator[] (int row)
	{return
		arr[row];
	}				
	int numrows () const//number of rows
	{return
		arr.size ();
	}
	int numcols () const	//Number of columns
	{return
		numrows () arr[0].size (): 0;
	}
Private:
	vector<vector<t>>arr;
};

This is achieved.