C++11 and auto with Eigen

Do not use auto when using Eigen with C++11.

Details here.

I had a linear system that I wanted to solve like so

// A is 4x4, so the result should be a Vector4d
auto result = A.colPivHouseholderQr().solve(b)

I just wanted x, y and z, so naturally I did a

return Vector3d { result.x(), result.y(), result.z() };

All’s good. Works great. In debug builds.

In release builds however I got random inf and nan everywhere

// result is an innocent (0.5, 0.5, 0.5, 1)
double x = result.x()
// x is a devious nan

I spent 2 days debugging this, gave up, then stumbled upon the FAQ in Eigen’s docs. A big facepalm later I “fixed” it

Vector3d result = A.colPivHouseholderQr().solve(b)

Do not use auto when using Eigen with C++11.