Re: [eigen] Forward declarations

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 19.07.2011 17:27, David Lonie wrote:
> When I try that, AutoAlign, RowMajor, ColMajor, and
> EIGEN_DEFAULT_MATRIX_STORAGE_ORDER_OPTION are all undefined. I could
> swap out the integer values for the constants and have a fragile
> solution, but AIUI EIGEN_DEFAULT_MATRIX_STORAGE_ORDER_OPTION is
> build-specific and cannot be known without including Eigen. Should I
> assume that the class cannot be forward declared and start looking at
> my other options, or is there a trick to get around this?

I have not looked deeply into your library, but I am afraid that 
forward-declaration is doomed to fail as soon as someone actually 
includes the Eigen headers. IIRC forward-declaration for templated 
classes is not allowed to set the default parameters a second time.

What might work is to make a self-containing forward-declaration header 
for Eigen (similar to C++ standards <iosfwd>) which is also included by 
Eigen itself. This could be copied by libraries using it, however there 
would be no downwards compatibility then ...

Also, what you could do on side of your library, is to inherit from 
Eigen classes and just forward-declare the inheriting classes. However, 
this comes at the cost of needing to reimplement the constructors.


Christoph


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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
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