Thanks Christoph!
On Fri, Oct 4, 2019 at 4:20 AM Christoph Hertzberg <
[email protected]> wrote:
> 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;
>
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,
Luke
>
> 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
> >
>
> --
> Dr.-Ing. Christoph Hertzberg
>
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