Re: [eigen] Documenting functions which return an Matrix

[何震邦 <jdh8@xxxxxxxxxxxxxx>]

You can also use a functor.

If you are trying to manipulate different sizes of matrices, I suggest you use
MatrixBase, the base class of dense matrix expression.

We use CRTP to implement static polymorphism. For example, MatrixXd derives
from MatrixBase<MatrixXd>.

template<typename ResultType>
struct my_functor
{
  static ResultType run(const MatrixBase<Derived> &_A) const
  {
    const Derived &A = _A;
    // play with A;
    // return something;
  }
}

Later, you can use it like:

MatrixXd A, B;
A = my_functor<Matrix<double, 3, 6, RowMajor> >::run(B);

在 2013 二月 13 週三 12:52:35，Dale Lukas Peterson 寫道:
> I have some functions which need to return Matrices of specific sizes
> and I would like clients of the class to see at a glance what size
> those matrices are.  I have been doing things like:
> 
> Matrix<double, 3, 6, RowMajor> my_func();
> 
> then in client code:
> 
> a = my_func();
> 
> The reason I am declaring the function this way is primarily so users
> who see it know what size matrix they are getting back.  Is this the
> preferred way to do this?
> 
> Some of my matrices are small (3x3), and some are bigger (20x20).
> Inside of my_func(), I typically declare a matrix on the heap to do
> the necessary calculations and populate the matrix, and then return
> it.  I am fine with all things being on the heap, but it would be nice
> to know that I'm not copying things unnecessarily because of the way
> I've declared the function.

Passing const references is the best way to send invariant matrices into a
function. :)

> I realize I can require clients to pass a const reference to a Matrix
> of their choosing (Dynamic or Fixed) as a function parameter, rather
> than using return, but the syntax of operator= is nicer and I don't
> really want to give up that syntax.
> 
> 1) How do people document their functions that return matrices of
> known size, even if those matrices are allocated on the heap?
> 
> 2) I see four cases that I'm not 100% clear on what happens when I do
> something like a = my_func():
> 
> a) a is a dynamic sized Matrix and my_func returns a fixed size Matrix.
> b) a is a dynamic sized Matrix and my_func returns a dynamic size Matrix.
> c) a is a fixed sized Matrix and my_func returns a fixed size Matrix.
> d) a is a fixed sized Matrix and my_func returns a dynamic size Matrix.

It is quite safe becuase my_func() evaluates into a temporary object.

> Can anybody clarify this for me and give me recommendations on how to
> make it clear to users what the size of the matrices are without
> shooting myself in the foot?
> 
> Thanks,
> Luke

I hope this helps you. ;)

Regards,
Chen-Pang