Re: [eigen] writing a function that takes Eigen::MatrixBase or Eigen::Block

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Thanks Christoph!

On Fri, Oct 4, 2019 at 4:20 AM Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On 03/10/2019 06.45, Dale Lukas Peterson wrote:
> Following the guidance here [0], I have:
> template <typename Derived>
> void test(const Eigen::MatrixBase<Derived>& a)
> {
>    // Use case 1:
>    Derived aa;
>    // do stuff with aa
>    // Use case 2:
>    Derived bb = a;
>    // do stuff with bb
> }
> If I create an Eigen::Matrix and call test like so, it works fine:
> Eigen::Matrix<double, 2, 4> A;
> A << 1, 2, 3, 4,
>          5, 6, 7, 8;
> // Works
> test(A);

This works, because `Derived = Matrix<double, 2, 4>` in this case, which
is a valid standalone object.

> However, the following doesn't work:
> test(A.block<2, 1>(0, 1));

In this case `Derived` is something like `Block<Matrix<double,2,4>,2,1>`
which is not usable as a stand-alone object. It can only be constructed
from an existing Matrix object where it points to.

> However, the following does work:
> test(A.block<2, 1>(0, 1).eval());

The result of `.eval()` is again a `Matrix` object (should be

> Is .eval() the right approach here? Should I being doing something else at
> the call site, or in my function definition to better handle this? Does the
> return value of `.block<2, 1>(0, 1)` constitute a "plain matrix or vector
> (not an _expression_)" [1] or not, so that I can be certain this isn't
> creating a useless copy?

`.block<..>(..)` will always return a "free" view into an existing
object (or actually a block of another _expression_), `anything.eval()`
will always return a `Matrix<...>` (or `Array<...>` object) which will
be a copy, unless `anything` already was a plain Matrix or Array (in
that case it will return a const reference).

If you want to have a modifiable plain matrix object inside `void
test(...)` with the same size as `MatrixBase<Derived>`, you can write

   typedef typename Derived::PlainObject Mat;
   Mat aa;

I ended up doing this as:
Eigen::Matrix<typename Derived::Scalar, Derived::RowsAtCompileTime, Derived::ColsAtCompileTime> aa;

However I like your solution better since it is shorter and presumably it also captures the Options, MaxRows, and MaxCols template parameters correctly from Derived, whereas what I did will use whatever the defaults are configured to be.

Thanks again,

Usage demo:


> Sincerely,
> Luke
> [0]
> [1]

  Dr.-Ing. Christoph Hertzberg

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