How to port a grid to copyable entities and intersections

In Dune 2.4, the EntityPointer has been deprecated and a number of interface objects have become copyable and / or default-constructible. This page contains a list of the required changes for existing grids to help grid maintainers:

  • Entities and intersections must be copyable (and, if at all possible, also movable). Here, it is especially important to make sure that internal state doesn’t get shared between copies by accident and that no state is kept in the EntityPointer, EntityIterator or IntersectionIterator. There are new tests in the grid check that verify these requirements.

  • EntityIterators and IntersectionIterators must be default-constructible (by extension, this also includes the EntityPointer). Default-constructibility makes those iterators more compatible with STL iterators.

  • Entities and intersections must be default-constructible. The exact semantics were voted on in Doodle 29:
    Entities and intersections are default-constructible. A default-constructed entity is invalid. Invalid entities may be copied around, moved and compared, but using them in any other context leads to undefined behavior. If a valid entity is copied / moved into a invalid entity, the target entity becomes valid and vice versa. An invalid entity always compares false to all valid entities, and all invalid entities compare as equal. Intersections behave the same.

  • All methods that currently return an EntityPointer have to return an Entity instead. It is extremely important to return a fully constructed facade class from you implementation, otherwise the backwards compatibility helpers in the facades will break! So in YaspGrid you must return a Dune::Entity<...,YaspEntity> and not a naked YaspEntity. Keep this in mind, as a lot of grid implementations used to rely on the facade to automatically add the wrapper. The following methods have to be changed:

    • Entity::father() and Entity::subEntity<codim>() for entities of codimension 0
    • Intersection::inside() and Intersection::outside()
    • Grid::entityPointer(const EntitySeed&). This method has to be deprecated and you must add a new method Grid::entity(const EntitySeed&) that returns an entity object
  • Depending on your performance trade-offs, your entity and intersection iterators may now return either a const reference to an internally stored entity / intersection instance (good if those objects are expensive) or a newly constructed temporary (good for meta grids).

  • Important: While most of the infrastructure continues to work for non-ported grids, meta grids like IdentityGrid and GeometryGrid can only be used with host grids that provide the new interface.

  • Until your grid has been ported, you can add the define -DDUNE_GRID_CHECK_USE_DEPRECATED_ENTITY_AND_INTERSECTION_INTERFACE to your gridcheck targets. That way, the grid check will be compiled with the old interface (and create lots and lots of deprecation warnings).

Meta grids

If you are porting a meta grid, you should probably invest a little more effort to benefit from the meta grid specific improvements like being able to return temporaries and not having to juggle around with EntityPointers inside your meta entities. Take a look at the changes for IdentityGrid for inspiration.

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