Created attachment 546 [details]
Patch to the move constructor of DenseStorage

I noticed that the move constructor of DenseStorage does not ensure that the moved-from object is in a valid state.

Example: 

  #include <utility>
  #include <iostream>

  #include <eigen3/Eigen/Dense>

  int main()
  {
      const std::size_t n = 12;

      Eigen::MatrixXd source = Eigen::MatrixXd::Zero(n, n);
      Eigen::MatrixXd target = std::move(source);
      std::cout << source << std::endl;

      return 0;
  }

This program crashes on:
gcc version 4.9.2 (Ubuntu 4.9.2-0ubuntu1~14.04) 
passing the compiler flag -std=c++14

The reason is that Eigen::MatrixXd, as well as many other classes, are using DenseStorage to store their data. The move constructor of DenseStorage however has a bug. When DenseStorage A is moved into DenseStorage B, the data pointer of A is correctly set to nullptr, but the number of columns and number of rows is not.

According to the C++ standard, moved-from objects in the standard library 
"shall be placed in a valid but unspecified state." (http://stackoverflow.com/a/7028318/1097451) I believe that this is reasonable also for the Eigen library. I made a small patch and tested the above mentioned code. After applying the patch, the code correctly outputs nothing and does not crash.