I have a templated function taking in Eigen matrices that works great,

template<typename T>
void foo(const Eigen::Matrix<var<T>, Eigen::Dynamic, 1>& v1,
const Eigen::Matrix<var<T>, Eigen::Dynamic, 1>& v2) {
...
}

In particular, T gets deduced from the arguments so calling the function
is nice and clean.

Now I was playing with extending the function to be compatible with expression
templates via http://eigen.tuxfamily.org/dox/TopicFun ... Types.html,

template<typename T, typename DerivedA, typename DerivedB>
void foo(const Eigen::MatrixBase<DerivedA>& v1,
const Eigen::MatrixBase<DerivedA>& v2) {
...
}

This works as expected, but T can no longer be deduced through the
expression templates which makes calling the function a bit of a pain.
Is there anyway to deduce T here or will the expression templates
forever hide that info?

Thanks.

All MatrixBase<X> types have a typedef Scalar which exposes the underlying scalar. Thus, if you have

Code: Select all

typedef typename Eigen::MatrixBase<DerivedA>::Scalar Scalar;


at the top of your function you can then declare a variable of the correct type with "Scalar x;". In fact, many functions in Eigen use this.

In your case, Scalar would be var<T> so your var type needs to expect the underlying T in some way.

Great, thanks. We were able to make this work using Eigen::MatrixBase<DerivedA>::Scalar
and some enable_if templating magic to get the signatures right.

Tip: "typename DerivedA::Scalar" is shorter.