Copy templated function argument in Eigen

Question

I am writing a generic class that utilizes Eigen data types. I already have problems assigning constructor arguments to class member variables. A simplified version of my code would be:

template <typename Derived>
class A
{
public:
  Eigen::Matrix<Derived> M; // error C2976: too few template parameters

A(const Eigen::DenseBase<Derived> & V)
{
  M = V.eval(); // I would want to snapshot the value of V.
}
};

My question is now what data type M should be? I tried a variety of options, such as:

Eigen::internal::plain_matrix_type_column_major<Derived> M;
Eigen::DenseBase<Derived> M;

but they just generate different errors. Note that I use C++17 and expect the class template parameter to be inferred from the constructor.

Answer 1

The declaration of the Eigen::Matrix variable M should be something like:

Eigen::Matrix<typename Derived::Scalar, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> M; 

Then your code will compile. See Demo .

The detailed description of each of the template parameters can be found at this link .

Answer 2

A generic way of declaring your type would be to use the declaration generating it as a source, this way you don't have to look at specific ways to declare complicated template types, this is an example based on your code:

decltype(static_cast<Eigen::DenseBase<Derived> *>(nullptr)->eval()) M;

don't worry there is no nullptr dereference here because the code inside decltype is not executed.

As pointed in the comments there's a cleaner way of writing this:

decltype(declval<Eigen::DenseBase<Derived>>().eval()) M;

and if you're worried that the type might be a reference and don't want that:

remove_reference_t<decltype(declval<Eigen::DenseBase<Derived>>().eval())> M;

also don't forget to #include <type_traits> and either prefix everything with std:: or add using namespace std; to your code.

To make the syntax simpler for future use add this to the beginning of your code:

template<typename T, typename T::f>
using member_function_return_t = remove_reference_t<decltype(declval<T>().f())>;

and than declare the variable as:

member_function_return_t<Eigen::DenseBase<Derived>, Eigen::DenseBase<Derived>::eval> M;

Answer 3

Your container needs the actual "plain type" as template argument:

template <typename PlainType>
class A
{
    PlainType M; 
public:
    template<class Derived>
    A(const Eigen::MatrixBase<Derived> & V) : M(V) {}
};

And you need an additional template deduction rule:

template<class Derived>
A(const Eigen::MatrixBase<Derived> & V) -> A<typename Derived::PlainObject>;

Usage example ( on godbolt ):

template<class X>
void bar(X&); // just to read full type of A

void foo(Eigen::Matrix2d const& M)
{
    A a = M*M;
    bar(a);  // calls bar<A<Matrix2d>>();
}