Difference between revisions of "cpp/utility/functional/reference wrapper/operator cmp"
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< cpp | utility | functional | reference wrapper
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===Notes=== | ===Notes=== | ||
The return types of {{vl|4-6}} are [[cpp/language/template argument deduction#auto-returning functions|deduced from {{tt|return}} statements]] to avoid hard error when instantiating a {{c|std::reference_wrapper<T>}} with {{box|{{lti|cpp/standard library/synth-three-way|synth-three-way-result}}<T>}} being ill-formed. | The return types of {{vl|4-6}} are [[cpp/language/template argument deduction#auto-returning functions|deduced from {{tt|return}} statements]] to avoid hard error when instantiating a {{c|std::reference_wrapper<T>}} with {{box|{{lti|cpp/standard library/synth-three-way|synth-three-way-result}}<T>}} being ill-formed. | ||
| + | |||
| + | {{feature test macro|__cpp_lib_reference_wrapper|Comparisons for {{tt|std::reference_wrapper}}|value=202403L|std=C++26}} | ||
===Example=== | ===Example=== | ||
Latest revision as of 02:22, 8 July 2024
| friend constexpr bool operator==( reference_wrapper lhs, reference_wrapper rhs ); |
(1) | (since C++26) |
| friend constexpr bool operator==( reference_wrapper lhs, reference_wrapper<const T> rhs ); |
(2) | (since C++26) |
| friend constexpr bool operator==( reference_wrapper lhs, const T& ref ); |
(3) | (since C++26) |
friend constexpr auto operator<=>( reference_wrapper lhs, reference_wrapper rhs ); |
(4) | (since C++26) |
| friend constexpr auto operator<=>( reference_wrapper lhs, reference_wrapper<const T> rhs ); |
(5) | (since C++26) |
| friend constexpr auto operator<=>( reference_wrapper lhs, const T& ref ); |
(6) | (since C++26) |
Performs comparison operations on reference_wrapper objects.
1,2) Compares two
reference_wrapper objects. The objects compare equal if and only if lhs.get() and rhs.get() are equal.
- Both overloads participate in overload resolution only if lhs.get() == rhs.get() is well-formed and its result is convertible to bool.
- The overload (2) participates in overload resolution only if std::is_const_v<T> is false.
3) Compares
reference_wrapper object with a reference. The parameters compare equal if and only if lhs.get() is equal to ref.
- The overload (3) participates in overload resolution only if lhs.get() == ref is well-formed and its result is convertible to bool.
4) Compares two
reference_wrapper objects as if by synth-three-way(lhs.get(), rhs.get()). The return type is synth-three-way-result<T>. This overload participates in overload resolution only if the return type is well-formed.5) Compares two
reference_wrapper objects as if by synth-three-way(lhs.get(), rhs.get()). The return type is synth-three-way-result<T>. This overload participates in overload resolution only if std::is_const_v<T> is false and the return type is well-formed.6) Compares
reference_wrapper object with a reference as if by synth-three-way(lhs.get(), ref). The return type is synth-three-way-result<T>. This overload participates in overload resolution only if the return type is well-formed.The <, <=, >, >=, and != operators are synthesized from operator<=> and operator== respectively.
Contents |
[edit] Parameters
| lhs, rhs | - | reference_wrapper object to compare
|
| ref | - | reference to compare to the reference_wrapper object
|
[edit] Return value
1,2) lhs.get() == rhs.get().
3) lhs.get() == ref.
4,5) synth-three-way(lhs.get(), rhs.get()).
6) synth-three-way(lhs.get(), ref).
[edit] Exceptions
Throws when and what the comparison throws.
[edit] Notes
The return types of (4-6) are deduced from return statements to avoid hard error when instantiating a std::reference_wrapper<T> with synth-three-way-result<T> being ill-formed.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_reference_wrapper |
202403L | (C++26) | Comparisons for std::reference_wrapper
|
[edit] Example
| This section is incomplete Reason: no example |
