MultiTypeBlockVector: Use std::common_type for field_type

This change should make the field_type more meaningful. As far as I see it should not break any application.

If this is fine I'll add something similar to `MultiTypeBlockMatrix".

I like this!

mentioned in commit patrick.jaap/dune-istl@baca4efc

added 1 commit

• baca4efc - Replace std::vector by Dune::DynamicVector in a unit test

Compare with previous version

I like the common_type approach, but I'm wondering about the exact semantics. We should document how it works.

What is the field_type of e.g. MultiTypeBlockVector<DynamicVector<FieldVector<double,3>>,DynamicVector<FieldVector<float,1>>>?

Do we get a meaningful type? If not, we should ensure that we get a static error.

mentioned in commit patrick.jaap/dune-istl@86dda7dc

added 2 commits

• e5bdddbb - MultiTypeBlockVector: Use std::common_type for field_type
• 86dda7dc - Replace std::vector by Dune::DynamicVector in a unit test

Compare with previous version

@christi I added a test for some simple double and float combinations.

mentioned in commit patrick.jaap/dune-istl@876a5440

added 7 commits

• 86dda7dc...576775c4 - 4 commits from branch core:master
• 5e4d9dbc - MultiTypeBlockVector: Use std::common_type for field_type
• 876a5440 - Replace std::vector by Dune::DynamicVector in a unit test
• fa4cbd1d - MultiTypeBlockVector: Add static error message if no std::common_type exists

Compare with previous version

added 1 commit

• 00674d8e - MultiTypeBlockVector: Add static error message if no std::common_type exists

Compare with previous version

added 1 commit

• de10df78 - MultiTypeBlockVector: Add static error message if no std::common_type exists

Compare with previous version

added 1 commit

• fb8d4fd7 - MultiTypeBlockVector: Add static error message if no std::common_type exists

Compare with previous version

Thanks @carsten.graeser for the hints. I included all of them in the current update.

If you trigger a static_assertion anyway, I don't see any reason to define NoCommonType and CommonTypeEnabled. This could be replaced by

static_assert(Dune::is_detected_v<std::common_type_t, T...>, ...);

If you want to represent that there is no common type, you can use

using field_type = Dune::detected_t<std::common_type_t, T...>;

which is either std::common_type_t<T...> or Dune::nonesuch.

Both rely on (Dune-implementation of) the detected mechanism from TSv2, and are thus probably more idiomatic.

And if you want a custom fallback type:

using field_type = Dune::detected_or_t<NoCommonType, std::common_type_t, T...>;

That's a nice solution

Yes this looks indeed nice. how should we deal with the sizeof...(Args) == 0 case? Should I leave the std::conditional_t in or should be specialize an empty class MultiTypeBlockVector<>?

I would add a static_assert(sifeof...(Args) >0) unless there is a good reason to support this. Otherwise, the proposed solution would simply detect the fallback type, which is IMO still the best solution for an empty matrix.

changed this line in version 8 of the diff

Ok, I updated the MR and included the Std::detected_t solution. While adding the static_assert(sifeof...(Args) >0) assertion I got errors in almost all unit tests that the assertion fails. I really don't know why since no empty vectors are created.

added 1 commit

• b552a3ac - MultiTypeBlockVector: Add static error message if no std::common_type exists

Compare with previous version

added 1 commit

• cb6aea69 - MultiTypeBlockVector: adapt FieldTraits to the new field_type

Compare with previous version