reference_wrapper
Document #: | P2944R0 |
Date: | 2023-07-08 |
Project: | Programming Language C++ |
Audience: |
LEWG |
Reply-to: |
Barry Revzin <barry.revzin@gmail.com> |
Typically in libraries, wrapper types are comparable when their underlying types are comparable. tuple<T>
is equality comparable when T
is. optional<T>
is equality comparable when T
is. variant<T>
is equality comparable when T
is.
But reference_wrapper<T>
is a peculiar type in this respect. It looks like this:
template<class T> class reference_wrapper { public: // types using type = T; // [refwrap.const], constructors template<class U> constexpr reference_wrapper(U&&) noexcept(see below); constexpr reference_wrapper(const reference_wrapper& x) noexcept; // [refwrap.assign], assignment constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept; // [refwrap.access], access constexpr operator T&() const noexcept; constexpr T& get() const noexcept; // [refwrap.invoke], invocation template<class... ArgTypes> constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>); };
When T
is not equality comparable, it is not surprising that reference_wrapper<T>
is not equality comparable. But what about when T
is equality comparable? There are no comparison operators here, but nevertheless the answer is… maybe?
Because reference_wrapper<T>
is implicitly convertible to T&
and T
is an associated type of reference_wrapper<T>
, T
’s equality operator (if it exists) might be viable candidate. But it depends on exactly what T
is and how the equality operator is defined. Given a type T
and an object t
such that t == t
is valid, let’s consider the validity of the expressions ref(t) == ref(t)
and ref(t) == t
for various possible types T
:
T
|
ref(t) == ref(t)
|
ref(t) == t
|
---|---|---|
builtins | ✔️ | ✔️ |
class or class template with member == |
❌ | ✔️ (since C++20) |
class with non-member or hidden friend == |
✔️ | ✔️ |
class template with hidden friend == |
✔️ | ✔️ |
class template with non-member, template == |
❌ | ❌ |
std::string_view |
❌ | ✔️ |
That’s a weird table!
Basically, if T
is equality comparable, then std::reference_wrapper<T>
is… sometimes… depending on how T
’s comparisons are defined. std::reference_wrapper<int>
is equality comparable, but std::reference_wrapper<std::string>
is not. Nor is std::reference_wrapper<std::string_view>
but you can nevertheless compare a std::reference_wrapper<std::string_view>
to a std::string_view
.
So, first and foremost: sense, this table makes none.
Second, there are specific use-cases to want std::reference_wrapper<T>
to be normally equality comparable, and those use-cases are the same reason what std::reference_wrapper<T>
exists to begin with: deciding when to capture a value by copy or by reference.
Consider wanting to have a convenient shorthand for a predicate to check for equality against a value. This is something that shows up in lots of libraries (e.g. Björn Fahller’s lift or Conor Hoekstra’s blackbird), and looks something like this:
Which allows the nice-looking:
But this implementation always copies (or moves) the value into the lambda. For larger types, this is wasteful. But we don’t want to either unconditionally capture by reference (which sometimes leads to dangling) or write a parallel hierarchy of reference-capturing function objects (which is lots of code duplication and makes the library just worse).
This is exactly the problem that std::reference_wrapper<T>
solves for the standard library: if I want to capture something by reference into std::bind
or std::thread
or anything else, I pass the value as std::ref(v)
. Otherwise, I pass v
. We should be able to use the exact same solution here, without having to change the definition of equals
:
And this works! Just… only for some types, seemingly randomly. The goal of this proposal is for it to just always work.
Add ==
and <=>
to std::reference_wrapper<T>
so that std::reference_wrapper<T>
is always comparable when T
is, regardless of how T
’s comparisons are defined.
Change 22.10.6.1 [refwrap.general]:
template<class T> class reference_wrapper { public: // types using type = T; // [refwrap.const], constructors template<class U> constexpr reference_wrapper(U&&) noexcept(see below); constexpr reference_wrapper(const reference_wrapper& x) noexcept; // [refwrap.assign], assignment constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept; // [refwrap.access], access constexpr operator T& () const noexcept; constexpr T& get() const noexcept; // [refwrap.invoke], invocation template<class... ArgTypes> constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>); + // [refwrap.comparisons], comparisons + friend constexpr bool operator==(reference_wrapper, reference_wrapper); + friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, reference_wrapper); };
Add a new clause, [refwrap.comparisons], after 22.10.6.5 [refwrap.invoke]:
We don’t have a feature-test macro for std::reference_wrapper<T>
, and there doesn’t seem like a good one to bump for this, so let’s add a new one to 17.3.2 [version.syn]