Eigen parameter in c++ function

Question

I use Eigen library in my C++ project and I don't understand how to pass an Eigen::Vector in parameter of my function.

I have in my main function :

Eigen::Vector<double, 3> direction_vector_ned = {1.0, 2.0, 3.0};

Eigen::Vector<double, 3> direction_vector_lla = Reperes::ned_to_lla(direction_vector_ned);

In my Reperes class :

class Reperes
{
  private:

  public:
    static Eigen::Vector<double, 3> ned_to_lla(const Eigen::Vector<double, 3>& ned);
};

And the implementation match with the prototype :

static Eigen::Vector<double, 3> ned_to_lla(const Eigen::Vector<double, 3>& ned)
{
    Eigen::Vector<double, 3> lla = {0.0, 0.0, 0.0};
    ...
    return lla;
}

I think the architecture is correct but the compiler raise an error at the call of ned_to_lla() in my main.

I got this error :

undefined reference to `Reperes::ned_to_lla(Eigen::Matrix<double, 3, 1, 0, 3, 1> const&)

I explicitly declared my direction_vector_ned variable as a Vector<double, 3>. Why it has been casted into a Matrix<double, 3, 1, 0, 3, 1> ?

I tried to use EigenBase type as mentionned in this section https://eigen.tuxfamily.org/dox/TopicFunctionTakingEigenTypes.html but it didn't work.

Matrix has default parameter, so Eigen::Vector<double, 3> IS Matrix<double, 3, 1, 0, 3, 1> — Jarod42
– Jarod42, Commented Jun 9 at 18:02
"undefined reference to Reperes::ned_to_lla" You define the free function ::ned_to_lla — Jarod42
– Jarod42, Commented Jun 9 at 18:03
You're right. I replaced the parameter with an int and got the same error. But I put the #include ‘reperes.hpp’ in my main.cpp — LnlB
– LnlB, Commented Jun 9 at 18:26

jjramsey · Accepted Answer · 2025-06-09 18:45:26Z

3

To expand on @Jarod42's comment, you never actually define Reperes::ned_to_lla. Instead, you define a free function that happens to have the same name as a member function of your Reperes class. This is NOT an implementation of Reperes::ned_to_lla:

static Eigen::Vector<double, 3> ned_to_lla(const Eigen::Vector<double, 3>& ned)
{
    Eigen::Vector<double, 3> lla = {0.0, 0.0, 0.0};
    ...
    return lla;
}

The above function does not belong to any class.

What you should have written was this:

Eigen::Vector<double, 3> Reperes::ned_to_lla(const Eigen::Vector<double, 3>& ned)
{
    Eigen::Vector<double, 3> lla = {0.0, 0.0, 0.0};
    ...
    return lla;
}

Notice the Reperes:: in front of the function name and no keyword static either (HT @gerum). That's what identifies it as part of your Reperes class and not some free function.

edited Jun 9 at 18:45

answered Jun 9 at 18:29

jjramsey

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8 Comments

LnlB Jun 9 at 18:38

If I do this, i got this

error: cannot declare member function 'static Eigen::Vector<double, 3> Reperes::ned_to_lla(Eigen::Vector<double, 3>&)' to have static linkage

jjramsey Jun 9 at 18:41

Does member function Reperes::ned_to_lla contain member variables of the Reperes class? Then it cannot be static.

LnlB Jun 9 at 18:49

No, it doesn't.

gerum Jun 9 at 18:41

@jjramsey The definition of the function should not contain the static keyword.

LnlB Jun 9 at 18:50

Yes, i resolved the problem like that. I thought the function prototypes had to match perfectly

jjramsey Jun 9 at 18:43

@gerum Good point. I'll update my post accordingly.

jjramsey Jun 9 at 18:50

@LnlB "I removed the static keyword in the .cpp file but i left it in the hpp" That's how you're supposed to do it: stackoverflow.com/a/5373143/1643973

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