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

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Hi!

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 
`Matrix<double,2,1>`).

> 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;

Usage demo: https://godbolt.org/z/hgaEra

Cheers,
Christoph



> Sincerely,
> Luke
>
> [0] https://eigen.tuxfamily.org/dox/TopicFunctionTakingEigenTypes.html
> [1] https://eigen.tuxfamily.org/dox/classEigen_1_1DenseBase.html#title39

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 Dr.-Ing. Christoph Hertzberg

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