dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Feb 08, 2010 at 01:35:16PM -0800, Johan Hake wrote:
> On Monday 08 February 2010 01:15:12 Anders Logg wrote:
> > On Sun, Feb 07, 2010 at 11:25:28PM +0000, Garth N. Wells wrote:
> > > Anders Logg wrote:
> > > > On Sun, Feb 07, 2010 at 10:58:44PM +0000, Garth N. Wells wrote:
> > > >> Anders Logg wrote:
> > > >>> This looks a like a non-optimal solution. Are you returning the mesh
> > > >>> by value? That will lead to creation of a temporary and copying of
> > > >>> the entire refined mesh.
> > > >>
> > > >> So they used say ;). Apparently modern compilers are all clever enough
> > > >> not to.
> > > >>
> > > >> Garth
> > > >
> > > > Is that really so? Do you have any good references that explain this
> > > > in detail?
> > >
> > >   http://en.wikipedia.org/wiki/Return_value_optimization
> > >
> > > and from 'man gcc'
> > >
> > > -fno-elide-constructors
> > >   The C++ standard allows an implementation to omit creating a temporary
> > > which is only used to initialize another object of the same type.
> > > Specifying this option disables that optimization, and forces G++ to
> > > call the copy constructor in all cases.
> >
> > ok, seems correct. I tried the following piece of code with some
> > debugging added to the copy constructor and assignment operator in
> > class Mesh:
> >
> >   Mesh refined_mesh_1 = UniformMeshRefinement::refine(mesh);
> >
> >   Mesh refined_mesh_2;
> >   refined_mesh_2 = UniformMeshRefinement::refine(mesh);
> >
> > The first refinement does not lead to any call to either of the copy
> > constructor or assignment operator.
> >
> > The second call leads to one call to the assignment operator.
> >
> > This is expected but means we need to be careful with how the
> > refinement is called (on the same line as initialization).
> >
> > Should we have a look and see if there are other places where we could
> > return by value?
>
> The operator* and operator+ were explicitly not included because they needed
> to be implemented as a return by value method. Now they can be added in the
> same way as the Python equivalents I think.

You mean in the linear algebra interface?

--
Anders


> Johan
>
> > We also need to add refine.h/refine.cpp and make refine() a free
> > function as discussed before to avoid dependending on adaptivity
> > library from the mesh class.
> >

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