Wrong results using auto with Eigen

Question

I got different results using auto and using Vector when summing two vectors.

My code:

#include \"stdafx.h\"
#include 
#         


        

Answer 1

The auto keyword tells the compiler to "guess" the best object based on the right hand side of the =. You can check the results by adding

std::cout << typeid(resAuto).name() <<std::endl;
std::cout << typeid(resVector3).name() <<std::endl;

to foo (don't forget to include <typeinfo>).

In this case, after constructing the temporary Vector3, the operator+ method is called, which creates a CwiseBinaryOp object. This object is part of Eigens lazy evaluation (can increase performance). If you want to force eager evaluation (and therefore type determination), you could use

const auto resAuto = (Ha + Vector3(0.,0.,j * 2.567)).eval();

instead of your line in foo.

A few side notes:

• Vector3 is identical to the Vector3d class defined in Eigen
• You can use #include <Eigen/Core> instead of #include <Eigen/Geometry> to include most of the Eigen headers, plus certain things get defined there that should be.