[Dune] Default constructor in MultiLinearGeometry

[Christian Engwer]

On Sun, Oct 14, 2012 at 03:33:17PM +0200, 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 :-) ]

It is true, we can work around this problem and it shouln't actually
be a problem... slight changes in the code will do.

So a default-constructor would just be a convience thing.

I don't have a strong opinion concerning the default constructor.

Christian

> 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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-- 
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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