Strange error in templated constructor

Question

I am getting a strange compiler error when trying to cast an argument in a templated constructor. Here is a minimal example:

#include <Eigen/Dense>

class Plane
{
public:
    template <typename TVector>
    Plane(const TVector& coefficients) {
        coefficients.cast<double>(); // compiler error on this line
    }

// This compiles fine
//    Plane(const Eigen::Vector3d& coefficients) {
//        coefficients.cast<double>();
//    }
};

int main(){return 0;}

The error is:

expected primary-expression before 'double'
expected ';' before 'double'

Since this class is never instantiated (main() is empty), I thought that the compiler wouldn't even see the function template at all, so I'm very confused as to how there is an error in this expression?

score 2 · Accepted Answer · answered Feb 11 '16 at 20:48

2

You have to use template keyword:

template <typename TVector>
Plane(const TVector& coefficients) {
    coefficients.template cast<double>();
}

answered Feb 11 '16 at 20:48

Jarod42

203,559
14
181
302

Yep, that does it. Can you say what this is called so I can look up when I need to use it? – David Doria Feb 11 '16 at 20:50
@DavidDoria He did. It is the [`template` keyword](http://en.cppreference.com/w/cpp/keyword/template) – clcto Feb 11 '16 at 20:51
@DavidDoria see the link above your original question, which has been marked as duplicate. The link explains this phenomenon in full. – iksemyonov Feb 11 '16 at 21:08

Strange error in templated constructor

1 Answers1