c++ - Eigen's matrix seems to preserve the memory in the different scope

Question

The following snippets, I thought that standard output must be the same in the both scope1 and scope2. However, as the results shows there are not. And the result implies that m in the scope2 inherits some values from the ones in scope1. I think this is strange because m in scoper1 is supposed to has nothing to do with m in scope2. Could someone explain why this happens?

#include<iostream>
#include<Eigen/Core>
using namespace Eigen;

int main(){
  {
    std::cout << "scope1" << std::endl; 
    MatrixXd m(2, 2);
    std::cout << m << std::endl; 
    m << 1, 2, 3, 4;
    std::cout << m << std::endl; 
  }
  {
    std::cout << "scope2" << std::endl; 
    MatrixXd m(2, 2);
    std::cout << m << std::endl; 
    m << 1, 2, 3, 4;
    std::cout << m << std::endl; 
  }
}

output

scope1
0 0
0 0
1 2
3 4
scope2
0 2
3 4
1 2
3 4

Answer 1

The documentation says:

Constructs an uninitialized matrix with rows rows and cols columns.

A program that reads uninitialized data has undefined behaviour.