Re: C++ Question

[Paul Kienzle]

Quentin H. Spencer wrote:

> Hello all,
>
> I have been writing some functions in C that use the octave ComplexSVD 
> class, and I'm having trouble understanding some strange behavior. 
> Initially I wrote the function with the following syntax:
>     
>     ComplexSVD svd;
>     svd = ComplexSVD( matrix , SVD::std);
>     ComplexMatrix U    = svd.left_singular_matrix(); 

This is a copy constructor

> Later I changed the syntax to the following (which mirrors the syntax 
> used in svd.cc).
>
>     ComplexSVD svd( matrix , SVD::std); 

This is a constructor.

>     ComplexMatrix U    = svd.left_singular_matrix();
>
> When using the first syntax, the function would usually return 
> properly the first time, but subsequent runnings of my C++ function 
> (and sometimes even octave's builtin svd function) often resulted in 
> the following:
>
> error: ComplexSVD: U not computed because type == SVD::sigma_only
>
> This is usally followed by a segmentation fault. There could be some 
> technicality about C++ that I don't understand here, but I don't see 
> the different between these two implementations. Can anyone enlighten me?
The copy constructor code in liboctave/CmplxSVD.h, which is the function
  ComplexSVD& operator= (const ComplexSVD& a)
does not copy the type_computed variable, so it keeps the value it had
before the copy.  Since your svd is uninitalized it is assumes its default
value (which may be random --- I don't know what C++ does with
uninitialized enums).

Copy it, test it and send a patch to bug-octave.

Thanks,

Paul Kienzle
address@hidden



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