[Dune-devel] DenseVector, FieldVector: a couple of small changes

[Ansgar Burchardt]

Hi,

I was looking at DenseVector to rename some arguments[1].  While doing
so I noticed several things:

a) Why is `Traits::derived_type` used instead of just `V`?
Note that `asImp()` returns `V&` and is used to return a
`derived_type&`.

I consider adding a static_assert to ensure V == derived_type and maybe
stop using Traits::derived_type in the future.

b) Not all variants of `operator=` return a `derived_type&`.  This
looks like an oversight and I plan to change that.

c) Why is FieldVector's constructor copying from a FieldVector (with
different field and size) explicit, but the one copying from a generic
DenseVector not?

d) I would like to explicitly delete functions instead of having
static_asserts, that is:

```c++
template<typename T, int N>
struct FieldVector
{
        FieldVector() = default;
        FieldVector(const FieldVector&) = default;

        template<typename T2>
        FieldVector(const FieldVector<T2, N>&) {}

        template<typename T2, int N2>
        FieldVector(const FieldVector<T2, N2>&) = delete;
};      

void f()
{       
        FieldVector<double, 2> x1;
        FieldVector<float, 2> x2(x1);  // OK
        FieldVector<double, 3> x3(x1); // error: use of deleted function
        FieldVector<float, 3> x4(x1);  // error: use of deleted function
}
```

instead of having only one template function with template arguments
(T2, N2) and a static_assert to make sure N2==N.

(Just not having it would mean the generic version for a DenseVector
would be used which is also wrong.)

This makes the interface explicitly state that N2==N is required
instead of having it hidden in a static_assert in the function
definition.

Ansgar

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