Do rvalue references to const have any use?

问题

I guess not, but I would like to confirm. Is there any use for const Foo&&, where Foo is a class type?

回答1:

They are occasionally useful. The draft C++0x itself uses them in a few places, for example:

template <class T> void ref(const T&&) = delete;
template <class T> void cref(const T&&) = delete;

The above two overloads ensure that the other ref(T&) and cref(const T&) functions do not bind to rvalues (which would otherwise be possible).

Update

I've just checked the official standard N3290, which unfortunately isn't publicly available, and it has in 20.8 Function objects [function.objects]/p2:

template <class T> void ref(const T&&) = delete;
template <class T> void cref(const T&&) = delete;

Then I checked the most recent post-C++11 draft, which is publicly available, N3485, and in 20.8 Function objects [function.objects]/p2 it still says:

template <class T> void ref(const T&&) = delete;
template <class T> void cref(const T&&) = delete;

回答2:

They are allowed and even functions ranked based on const, but since you can't move from const object referred by const Foo&&, they aren't useful.

回答3:

I can't think of a situation where this would be useful directly, but it might be used indirectly:

template<class T>
void f(T const &x) {
  cout << "lvalue";
}
template<class T>
void f(T &&x) {
  cout << "rvalue";
}

template<class T>
void g(T &x) {
  f(T());
}

template<class T>
void h(T const &x) {
  g(x);
}

The T in g is T const, so f's x is an T const&&.

It is likely this results in a comile error in f (when it tries to move or use the object), but f could take an rvalue-ref so that it cannot be called on lvalues, without modifying the rvalue (as in the too simple example above).

回答4:

Besides std::ref, the standard library also uses const rvalue reference in std::as_const for the same purpose.

template <class T>
void as_const(const T&&) = delete;

It is also used as return value in std::optional when getting the wrapped value:

constexpr const T&& operator*() const&&;
constexpr const T&& value() const &&;

As well as in std::get:

template <class T, class... Types>
constexpr const T&& get(const std::variant<Types...>&& v);
template< class T, class... Types >
constexpr const T&& get(const tuple<Types...>&& t) noexcept;

This is presumably in order to maintain the value category as well as constness of the wrapper when accessing the wrapped value.

This makes a difference whether const rvalue ref-qualified functions can be called on the wrapped object. That said, I don't know any uses for const rvalue ref qualified functions.

来源：https://stackoverflow.com/questions/4938875/do-rvalue-references-to-const-have-any-use

标签

c++

c++11

const

rvalue-reference