How do I identify whether an object is an Eigen type?

Question

Suppose I want to know whether a type is an Eigen object, as opposed to a primitive or some other class. How do I achieve this without actually casting the object?

template <typename Derived>
EigenBase<Derived> fun(const EigenBase<Derived>& value)
{
  // This is bad, because value is now of type EigenBase<Derived> and I don't know
  // what the original type is.
  return value;
}

template <typename T>
T fun(const T& value)
{
  // This will also catch primitives and other classes.
  return value;
}

How can I catch only types which can be type-deduced to Eigen::EigenBase<T>, without actually casting them to that type?

Answer 1

In the first version, the actuel expression type is Derived, and you can get it via the .derived() member:

template <typename Derived>
typename Derived::PlainObject fun(const EigenBase<Derived>& a_value)
{
  const Derived &value(a_value.derived());     // the actual expression object
  typename Derived::PlainObject ret;           // this is an object with storage, typically the Matrix<> type closest to Derived.
  ret = 2*value;
  return ret;
}

Moreover you should never returns by value, and more generally create, an EigenBase<> object. EigenBase has no storage at all, it is like an abstract base class.