[Dune] Default constructor in MultiLinearGeometry

[Christian Engwer]

On Sun, Oct 14, 2012 at 04:03:26PM +0200, Martin Nolte wrote:
> Hi Oli,
> 
> the long compile time is due to the reference elements. In my branch
> it compiles within less than 30s. Let's discuss the reference
> element stuff, so that compile time goes down (for all of us); even
> though this means less time for fetching coffee ;-).

Yes, we should start again discussing these things :-)

Christian

> Best,
> 
> Martin
> 
> On 10/14/2012 03:33 PM, Oliver Sander wrote:
> >Hi,
> >
> >>of gcc, if I remember correctly). To fill an std::vector with a default
> >>value, you can always use
> >>
> >>const T default( your default arguments here );
> >>std::vector< T > myVector( size, default );
> >>
> >
> >I tried that and it seems compile.
> >['Seems to' because compiling test-multilineargeometry with an additional
> >line std::vector< T > myVector( size, default ); made my compiler run
> >longer than I was willing to wait, and using all my 4GB :-) ]
> >
> >Best,
> >Oliver
> >
> >PS: Let's not discuss this in FlySpray -- I don't think anyone but us three
> >is interested. I retire my wish for a default constructor. Christian,
> >what do you say?
> >
> >>If you fill the vector linearly, you can even use
> >>
> >>std::vector< T > myVector;
> >>myVector.reserve( size );
> >>for( std::size_t i = 0; i < size; ++i )
> >>myVector.push_back( T( arguments for element i ) );
> >>
> >>Anyway, attached you first version of a possible patch to allow
> >>uninitialized MultiLinearGeometry objects. I have not tested anything,
> >>it just shows what this could basically look like.
> >>
> >>Best,
> >>
> >>Martin
> >>
> >>
> >>On 10/14/2012 11:32 AM, Christian Engwer wrote:
> >>>Hi Martin,
> >>>
> >>>>Apart from this, my feeling is that uninitialized objects are a very
> >>>>bad thing. Especially unexperienced programmers will have a hard
> >>>>time finding bugs caused by this. A default constructor would, of
> >>>>course, create an "uninitialized" geometry (in the sense that you
> >>>>may not use it at all).
> >>>
> >>>while I agree, that RAII is a good idea, default constructors aren't a
> >>>bad thing in general. For example you won't be able to store objects
> >>>in a std::vector, unless they have a default constructor. This is due
> >>>to std::vectors' memory management, which tries to avoid
> >>>reallocations.
> >>>
> >>>So I think a defaults constructor would be helpful. E.g. in dune-udg
> >>>we construct a set of entity-parts (geometries + something) between a
> >>>grid entity and a levelset. These entity-parts are stored (on demand,
> >>>for a single intersection) in an std::vector -- we are doing nearly
> >>>the same as return a copy on the fly, but we have to return a set and
> >>>thus need std::vector. A default constructor would allow us to switch
> >>>to MultiLinearGeometry, allowing faster computations.
> >>>
> >>>In case the corner storage does not allow default-construction, we
> >>>still get a useful error message.
> >>>
> >>>Cheers
> >>>Christian
> >>>
> >>
> >>
> >>
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> >
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> 
> -- 
> Dr. Martin Nolte <nolte at mathematik.uni-freiburg.de>
> 
> Universität Freiburg                                   phone: +49-761-203-5630
> Abteilung für angewandte Mathematik                    fax:   +49-761-203-5632
> Hermann-Herder-Straße 10
> 79104 Freiburg, Germany
> 
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-- 
Prof. Dr. Christian Engwer 
Institut für Numerische und Angewandte Mathematik
Fachbereich Mathematik und Informatik der Universität Münster
Einsteinstrasse 62
48149 Münster

E-Mail	christian.engwer at uni-muenster.de
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