Re: [eigen] eigen sqrt compilation error

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On Fri, Jun 19, 2009 at 12:29 PM, Patrick Mihelich
<patrick.mihelich@xxxxxxxxx> wrote:
> On Fri, Jun 19, 2009 at 12:37 AM, Moritz Lenz <mlenz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>> Considering the number of times people make this error (forgetting to include it), what about including it by default? Is there a very good reason not to do it?
> I wonder this too. I ran into this error when I started using Eigen, and it seems to pop up regularly on the list. What exactly is the cost of including <Eigen/Array> in the core? How much does it increase compile times?

on my high end computer with gcc 4.3.2 -O2 -DNDEBUG, much less time
than to compile: Vector3f a, b; cout << a+b;, so something around 0.02

> Documentation is good, but it is still too easy to use the Eigen API incorrectly like this, and the error messages are no help.
> Awhile back I suggested a trick based on default arguments, which is a little ugly but does massively improve the error messages. Here it is again:
> // Changes to MatrixBase
> template<typename Derived> class MatrixBase
> {
>   private:
>     struct ERROR_YOU_MUST_INCLUDE_Eigen_Array {};
>   public:
>     const PartialRedux<Derived, Horizontal> rowwise(ERROR_YOU_MUST_INCLUDE_Eigen_Array) const;
> };
> // Changes to implementation in Array module
> // Defining the default argument here changes the function signature, allowing matrix.rowwise()
> template<typename Derived>
> const PartialRedux<Derived,Horizontal> MatrixBase<Derived>::rowwise(ERROR_YOU_MUST_INCLUDE_Eigen_Array = ERROR_YOU_MUST_INCLUDE_Eigen_Array()) const
> {
>   /* implementation */
> }
> Then when the user includes <Eigen/Array>, he effectively gets the expected zero-arg version of rowwise. If he forgets to include it, no zero-arg version exists, so mat.rowwise() should produce a compiler error along the lines of:
> error: no matching function for call to 'MatrixBase<Derived>::rowwise()'
> note: candidates are: const PartialRedux<Derived, 0> MatrixBase<Derived>::rowwise(MatrixBase<Derived>::ERROR_YOU_MUST_INCLUDE_Eigen_Array) const
> As usual you can wrap the dummy argument in a macro to selectively hide it from doxygen.
> Cheers,
> Patrick

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