Add utilities for working with `std::reference_wrapper`

This adds three utilities (stolen from staging/dune-functions!315 (merged)) and a corresponding test for working with std::reference_wrapper:

• IsReferenceWrapper_v<T> allows to check if T is an instantiation of std::reference_wrapper
• The function resolveRef(t) either returns t.get() or t depending on whether t is a std::reference_wrapper or a plain l-value reference.
• ResolveRef_t<T> provides the resolved type for T.

These utilities allow to support storing data members of classes by value or reference transparently suing the following pattern. The class always stores values, but supports passing std::ref(...) to opt-in store-by-reference. Handling the latter can be done without boilerplate code using the provided utilities:

template<class StoredT>
struct SomeWrapper {

  using T = ResolveRef_t<StoredT>;

  SomeWrapper(const StoredT& t) : t_(t) {}

  void callFoo() const {
    resolveRef(t_).foo();
  }

  StoredT t_;
};

[...]

// Store t by value
auto w1 = SomeWrapper(t);
w1.callFoo();

// Store t by reference
auto w2 = SomeWrapper(std::cref(t));
w2.callFoo();

mentioned in merge request !1101 (closed)

I'm really supporting this. I'm using this reference wrapper utility quiet a lot recently. It makes storing reference more explicit.

What do you think could be a "best-practice" when passing the stored object to the wrapper class? You have passed to the constructor by const&. An alternative could be:

SomeWrapper(StoredT t) : t_(std::move(t)) {}

My motivation for writing it like this is: It tells the user that the passed object is stored by value right away in the constructor.

That's a good argument. But as far as I know this case is neither listed for guaranteed nor for optional copy-elision. Hence we have to hope for compiler optimization which may not work in the case of side effects. Thus I personally tend to pass references. But maybe I'm to worried here.

Does pass-by-reference still allows copy-elision? I don't know, maybe I should make some experiments.

added 1 commit

• da4bd702 - Add utilities for working with `std::reference_wrapper`

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resolved all threads

mentioned in merge request dune-grid!577 (merged)

Any objections to merging this?

merged

mentioned in commit fc637911

requested review from @simon.praetorius