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#include <Eigen/Core>
#include <iostream>
using namespace Eigen;
using namespace std;
template<typename Derived>
Eigen::Block<Derived, 2, 2>
topLeft2x2Corner(MatrixBase<Derived>& m)
{
return Eigen::Block<Derived, 2, 2>(m.derived(), 0, 0);
}
template<typename Derived>
const Eigen::Block<const Derived, 2, 2>
topLeft2x2Corner(const MatrixBase<Derived>& m)
{
return Eigen::Block<const Derived, 2, 2>(m.derived(), 0, 0);
}
int main(int, char**)
{
Matrix3d m = Matrix3d::Identity();
cout << topLeft2x2Corner(4*m) << endl; // calls the const version
topLeft2x2Corner(m) *= 2; // calls the non-const version
cout << "Now the matrix m is:" << endl << m << endl;
return 0;
}
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