Effect of operator<=> on the C++ Standard Library

Document Number: P0790R2
Date: 2019-01-21
Author: David Stone (davidmstone@google.com, david@doublewise.net)
Audience: LEWG, LWG

This paper lists (what are expected to be) non-controversial changes to the C++ standard library in response to P0515, which adds operator<=> to the language. This is expected to be non-controversial because it tries to match existing behavior as much as possible. As a result, all proposed additions are either strong_equality or strong_ordering, matching the existing comparison operators.

This document should contain a complete list of types or categories of types in C++.

Revision History

R1: A much broader version of this paper was presented to LEWG at a previous meeting. What remains in this paper is everything which the group did not find controversial and which probably does not require significant justification. All controversial aspects will be submitted in separate papers. R2: Added wording.

Backward Compatibility

The operator<=> proposal was written such that the "generated" operators are equivalent to source code rewrites – there is no actual operator== that a user could take the address of. Users are not allowed to form pointers to standard library member functions and are unable to form pointers to friend functions defined inline in the class. There are some cases where we do not specify how the operator was implemented, only that the expression a @ b is valid; these cases are not broken by such a change because users could not have depended on it, anyway. In general, we accept changes that overload existing functions, which also has the effect of breaking code which takes the address of a free function.

Types that are not proposed to get operator<=> in this paper

These types are not comparable now. This paper does not propose adding any new comparisons to any of these types.

• deprecated types
• exception types
• tag classes (nothrow, piecewise_construct_t, etc.)
• arithmetic function objects (plus, minus, etc.)
• comparison function objects (equal_to, etc.)
• owner_less
• logical function objects (logical_and, etc.)
• bitwise function objects (bit_and, etc.)
• nested_exception
• allocator_traits
• pool_options
• char_traits
• iterator_traits
• numeric_limits
• pointer_traits
• regex_traits
• chrono::duration_values
• tuple_element
• max_align_t
• map::node_type
• map::insert_return_type
• set::node_type
• set::insert_return_type
• unordered_map::node_type
• unordered_map::insert_return_type
• unordered_set::node_type
• unordered_set::insert_return_type
• any
• default_delete
• aligned_storage
• aligned_union
• system_clock
• steady_clock
• high_resolution_clock
• locale::facet
• locale::id
• ctype_base
• ctype
• ctype_byname
• codecvt_base
• codecvt
• codecvt_byname
• num_get
• num_put
• numpunct
• numpunct_byname
• collate
• collate_byname
• time_get
• time_get_byname
• time_put
• time_put_byname
• money_base
• money_get
• money_put
• money_punct
• moneypunct_byname
• message_base
• messages
• messages_byname
• FILE
• va_list
• back_insert_iterator
• front_insert_iterator
• insert_iterator
• ostream_iterator
• ostreambuf_iterator
• ios_base
• ios_base::Init
• basic_ios
• basic_streambuf
• basic_istream
• basic_iostream
• basic_ostream
• basic_stringbuf
• basic_istringstream
• basic_ostringstream
• basic_stringstream
• basic_filebuf
• basic_ifstream
• basic_ofstream
• basic_fstream
• gslice
• slice_array
• gslice_array
• mask_array
• indirect_array
• atomic_flag
• thread
• mutex
• recursive_mutex
• timed_mutex
• timed_recursive_mutex
• lock_guard
• scoped_lock
• unique_lock
• once_flag
• shared_mutex
• shared_timed_mutex
• shared_lock
• condition_variable
• condition_variable_any
• promise
• future
• shared_future
• packaged_task
• random_device
• hash
• weak_ptr
• basic_regex
• sequential_execution_policy
• parallel_execution_policy
• parallel_vector_execution_policy
• default_searcher
• boyer_moore_searcher
• boyer_moore_horspool_searcher
• ratio
• integer_sequence
• seed_seq (paper needed to add strong_equality)
• enable_shared_from_this: It would be nice to give it a strong_ordering to allow derived classes to = default. However, this means that all classes that do not explicitly delete their comparison operator get an operator<=> that compares only the enable_shared_from_this base class, which is almost certainly wrong. Since this is intended to be used as a base class, we should not add operator<=> to it. Moreover, classes which enable_shared_from_this are unlikely to be basic value classes so they do not lose much by not being able to default.
• initializer_list: initializer_list is a reference type. It would be strange to give it reference semantics on copy but value semantics for comparison. It would also be surprising if two initializer_list containing the same set of values compared as not equal. Therefore, I recommend not defining it for this type.

Types from C that are not proposed to get operator<=> in this paper

• div_t
• ldiv_t
• lldiv_t
• imaxdiv_t
• timespec
• tm
• lconv
• fenv_t
• fpos_t
• mbstate_t

Types that have only == and !=, and thus do not require <=>

• type_info
• bitset (paper would be needed to change this to strong_ordering)
• allocator
• memory_resource
• synchronized_pool_resource: (implicitly from memory_resource base class)
• unsynchronized_pool_resource: (implicitly from memory_resource base class)
• monotonic_buffer_resource: (implicitly from memory_resource base class)
• polymorphic_allocator
• scoped_allocator_adaptor
• function with nullptr_t only (no homogenous operator)
• locale
• complex (heterogeneous with T and homogeneous)
• linear_congruential_engine
• mersenne_twister_engine
• subtract_with_carry_engine
• discard_block_engine
• independent_bits_engine
• shuffle_order_engine
• uniform_int_distribution
• uniform_int_distribution::param_type
• uniform_real_distribution
• uniform_real_distribution::param_type
• bernoulli_distribution
• bernoulli_distribution::param_type
• binomial_distribution
• binomial_distribution::param_type
• geometric_distribution
• geometric_distribution::param_type
• negative_binomial_distribution
• negative_binomial_distribution::param_type
• poisson_distribution
• poisson_distribution::param_type
• exponential_distribution
• exponential_distribution::param_type
• gamma_distribution
• gamma_distribution::param_type
• weibull_distribution
• weibull_distribution::param_type
• extreme_value_distribution
• extreme_value_distribution::param_type
• normal_distribution
• normal_distribution::param_type
• lognormal_distribution
• lognormal_distribution::param_type
• chi_squared_distribution
• chi_squared_distribution::param_type
• cauchy_distribution
• cauchy_distribution::param_type
• fisher_f_distribution
• fisher_f_distribution::param_type
• student_t_distribution
• student_t_distribution::param_type
• discrete_distribution
• discrete_distribution::param_type
• piecewsie_constant_distribution
• piecewsie_constant_distribution::param_type
• piecewise_linear_distribution
• piecewise_linear_distribution::param_type
• istream_iterator
• istreambuf_iterator
• filesystem::path::iterator
• filesystem::directory_iterator
• filesystem::recursive_directory_iterator
• match_results
• regex_iterator
• regex_token_iterator
• fpos
• forward_list::iterator
• list::iterator
• map::iterator
• set::iterator
• multimap::iterator
• multiset::iterator
• unordered_map::iterator
• unodered_set::iterator
• unordered_multimap::iterator
• unodered_multiset::iterator

Types that should get <=> with a return type of strong_ordering, no change from current comparisons

These types are all currently comparable.

• error_category
• error_code
• error_condition
• exception_ptr
• monostate
• chrono::duration: heterogeneous with durations of other representations and periods
• chrono::time_point: heterogeneous in the duration
• type_index
• filesystem::path
• filesystem::directory_entry
• thread::id
• array::iterator
• deque::iterator
• vector::iterator
• valarray::iterator

Types that will get their operator<=> from a conversion operator

These types will get operator<=> if possible without any changes, just like they already have whatever comparison operators their underlying type has.

• integral_constant and all types deriving from integral_constant (has operator T)
• bitset::reference (has operator bool)
• reference_wrapper (has operator T &)
• atomic (has operator T)

This has the disadvantage that types which have a template comparison operator will not have their wrapper convertible. For instance, std::reference_wrapper<std::string> is not currently comparable. This does not affect bitset::reference, as it has a fixed conversion to bool, but it does affect the other three.

Types that wrap another type

• array
• deque
• forward_list
• list
• vector (including vector<bool>)
• map
• set
• multimap
• multiset
• unordered_map
• unodered_set
• unodered_multimap
• unordered_multiset
• queue
• queue::iterator
• priority_queue
• priority_queue::iterator
• stack
• stack::iterator
• pair
• tuple
• reverse_iterator
• move_iterator
• optional
• variant

This turned out to be much more complicated than expected and will require its own paper.

basic_string, basic_string_view, char_traits, and sub_match

Properly integrating operator<=> with these types requires more thought than this paper has room for, and thus will be discussed separately.

unique_ptr and shared_ptr

They contain state that is not observed in the comparison operators. Therefore, they will get their own paper.

valarray

Current comparison operators return a valarray<bool>, giving you the result for each pair (with undefined behavior for differently-sized valarray arguments). It might make sense to provide some sort of function that returns valarray<comparison_category>, but that should not be named operator<=>. This paper does not suggest adding operator<=> to valarray.

Types that have no comparisons now but are being proposed to get operator<=> in another paper

This paper does not propose changing any of the following types -- they are here only for completeness.

• filesystem::file_status
• filesystem::space_info
• slice
• to_chars_result
• from_chars_result

nullptr_t

Already supports strong_equality in the working draft. I will be writing a separate paper proposing strong_ordering.

Not Updating Concepts That Provide Comparisons

This category includes things like BinaryPredicate and Compare. This is addressed in a separate paper.

Not Updating Concepts That Require Comparisons

This includes things like LessThanComparable and EqualityComparable. This is addressed in a separate paper.

Miscellaneous

All operator<=> should be constexpr and noexcept where possible, following the lead of the language feature and allowing = default as an implementation strategy for some types.

When we list a result type as "unspecified" it is unspecified whether it has operator<=>. There are not any unspecified result types for which we currently guarantee any comparison operators are present, so there is no extra work to do here.

Wording

All wording is relative to N4791.

General wording

15.4.2.3 Operators [operators]

1 In this library, whenever a declaration is provided for an operator!=, operator>, operator<=, or operator>= for a type T, its requirements and semantics are as follows, unless explicitly specified otherwise.

bool operator!=(const T& x, const T& y);

2 Requires: Type T is Cpp17EqualityComparable (Table 23).

3 Returns: !(x == y).

bool operator>(const T& x, const T& y);

4 Requires: Type T is Cpp17LessThanComparable (Table 24).

5 Returns: y < x.

bool operator<=(const T& x, const T& y);

6 Requires: Type T is Cpp17LessThanComparable (Table 24).

7 Returns: !(y < x).

bool operator>=(const T& x, const T& y);

8 Requires: Type T is Cpp17LessThanComparable (Table 24).

9 Returns: !(x < y).

1 Unless specified otherwise, if lhs and rhs are values of types from this library, the following shall hold:

• lhs != rhs is a valid expression if and only if lhs == rhs is a valid expression.
• lhs < rhs, lhs > rhs, lhs <= rhs, and lhs >= rhs are all valid expressions if and only if lhs <=> rhs is a valid expression.
• If lhs <=> rhs is a valid expression, lhs == rhs is a valid expression.

The requirements and semantics of these operators are as follows, unless explicitly specified otherwise.

| Expression | Return type | Operational semantics | | ---------- | ---------- | ---------- | | lhs <=> rhs | std::strong_ordering | If lhs <=> rhs yields std::strong_ordering::equal, the two objects have an equivalence relation as described in Cpp17EqualityComparable ([utility.arg.requirements]). Regardless of the return value, lhs <=> rhs is a total ordering relation (24.7). | | lhs == rhs | Convertible to bool | lhs <=> rhs == 0 | | lhs != rhs | Convertible to bool | !(lhs == rhs) | | lhs < rhs | Convertible to bool | lhs <=> rhs < 0 | | lhs > rhs | Convertible to bool | lhs <=> rhs > 0 | | lhs <= rhs | Convertible to bool | lhs <=> rhs <= 0 | | lhs >= rhs | Convertible to bool | lhs <=> rhs >= 0 |

error_category

18.5.2.3 Non-virtual members [syserr.errcat.nonvirtuals] constexpr errorcategory() noexcept; 1 Effects: Constructs an object of class errorcategory.

bool operator==(const errorcategory& rhs) const noexcept; 2 Returns: this == &rhs. bool operator!=(const errorcategory& rhs) const noexcept; 3 Returns: !(*this == rhs). bool operator<(const error_category& rhs) const noexcept; 4 Returns: less<const error_category*>()(this, &rhs). [Note: less (19.14.7) provides a total ordering for pointers. — end note] 18.5.2.4 Comparisons [syserr.errcat.comparisons]

For two values lhs and rhs of type error_category, the expression lhs <=> rhs is of type std::strong_­ordering. If the address of lhs and rhs compare equal ([expr.eq]), lhs <=> rhs yields std::strong_­ordering::equal; if lhs and rhs compare unequal, lhs <=> rhs yields std::strong_­ordering::less if std::less{}(&lhs, &rhs) is true ([comparisons]), otherwise yields std::strong_ordering::greater.

18.5.2.45 Program-defined classes derived from error_category [syserr.errcat.derived]

[...]

18.5.2.56 Error category objects [syserr.errcat.objects]

error_code and error_condition

18.5.5 Comparison functions [syserr.compare] bool operator==(const errorcode& lhs, const errorcode& rhs) noexcept;

1 Returns: lhs.category() == rhs.category() && lhs.value() == rhs.value()

bool operator==(const error_code& lhs, const error_condition& rhs) noexcept;

2 Returns: lhs.category().equivalent(lhs.value(), rhs) || rhs.category().equivalent(lhs, rhs.value())

bool operator==(const error_condition& lhs, const error_code& rhs) noexcept;

3 Returns: rhs.category().equivalent(rhs.value(), lhs) || lhs.category().equivalent(rhs, lhs.value())

bool operator==(const error_condition& lhs, const error_condition& rhs) noexcept;

4 Returns: lhs.category() == rhs.category() && lhs.value() == rhs.value()

bool operator!=(const error_code& lhs, const error_code& rhs) noexcept;
bool operator!=(const error_code& lhs, const error_condition& rhs) noexcept;
bool operator!=(const error_condition& lhs, const error_code& rhs) noexcept;
bool operator!=(const error_condition& lhs, const error_condition& rhs) noexcept;

5 Returns: !(lhs == rhs).

bool operator<(const error_code& lhs, const error_code& rhs) noexcept;

6 Returns: lhs.category() < rhs.category() || (lhs.category() == rhs.category() && lhs.value() < rhs.value())

bool operator<(const error_condition& lhs, const error_condition& rhs) noexcept;

7 Returns: lhs.category() < rhs.category() || (lhs.category() == rhs.category() && lhs.value() < rhs.value()) For values lhs_code and rhs_code of type error_code and values lhs_condition and rhs_condition of type error_condition, the following holds:

1. The expression lhs_code <=> rhs_code is of type std::strong_­ordering. Equivalent to std::tie(lhs_code.category(), lhs_code.value()) <=> std::tie(rhs_code.category(), rhs_code.value()).
2. The expression lhs_condition <=> rhs_condition is of type std::strong_­ordering. Equivalent to std::tie(lhs_condition.category(), lhs_condition.value()) <=> std::tie(rhs_condition.category(), rhs_condition.value()).
3. The expression lhs_code == rhs_condition is of type bool. Equivalent to lhs_code.category().equivalent(lhs_code.value(), rhs_condition) || rhs_condition.category().equivalent(lhs_code, rhs_category.value()).

[Note: The function tie is defined in Clause 19.5. — end note]

monostate

19.7.9 monostate relational operators [variant.monostate.relops]

constexpr bool operator==(monostate, monostate) noexcept { return true; } constexpr bool operator!=(monostate, monostate) noexcept { return false; } constexpr bool operator<(monostate, monostate) noexcept { return false; } constexpr bool operator>(monostate, monostate) noexcept { return false; } constexpr bool operator<=(monostate, monostate) noexcept { return true; } constexpr bool operator>=(monostate, monostate) noexcept { return true; } For two values lhs and rhs of type monostate, the expression lhs <=> rhs is of type std::strong_­ordering and yields std::strong_­ordering::equal.

[Note: monostate objects have only a single state; they thus always compare equal. — end note]

type_index

19.17.2 type_index overview [type.index.overview]

namespace std {
    class type_index {
    public:
        type_index(const type_info& rhs) noexcept;

bool operator==(const typeindex& rhs) const noexcept; bool operator!=(const typeindex& rhs) const noexcept; bool operator< (const typeindex& rhs) const noexcept; bool operator> (const typeindex& rhs) const noexcept; bool operator<= (const typeindex& rhs) const noexcept; bool operator>= (const typeindex& rhs) const noexcept; sizet hashcode() const noexcept; const char* name() const noexcept; private: const type_info* target; // exposition only // Note that the use of a pointer here, rather than a reference, // means that the default copy/move constructor and assignment // operators will be provided and work as expected. }; }

1 The class type_index provides a simple wrapper for type_info which can be used as an index type in associative containers (21.4) and in unordered associative containers (21.5).

19.17.3 type_index members [type.index.members]

type_index(const type_info& rhs) noexcept;

1 Effects: Constructs a type_index object, the equivalent of target = &rhs.

bool operator==(const type_index& rhs) const noexcept;

2 Returns: *target == *rhs.target.

bool operator!=(const type_index& rhs) const noexcept;

3 Returns: *target != *rhs.target.

bool operator<(const type_index& rhs) const noexcept;

4 Returns: target->before(*rhs.target).

bool operator>(const type_index& rhs) const noexcept;

5 Returns: rhs.target->before(*target).

bool operator<=(const type_index& rhs) const noexcept;

6 Returns: !rhs.target->before(*target).

bool operator>=(const type_index& rhs) const noexcept;

7 Returns: !target->before(*rhs.target).

size_t hash_code() const noexcept;

82 Returns: target->hash_code().

const char* name() const noexcept;

92 Returns: target->name().

19.17.4 Hash support [type.index.hash]

template<> struct hash<type_index>;

1 For an object index of type type_index, hash<type_index>()(index) shall evaluate to the same result as index.hash_code().

19.17.5 Comparisons [type.index.comparisons]

For two values lhs and rhs of type type_index, the expression lhs <=> rhs is of type std::strong_­ordering. If *lhs.target == *rhs.target, lhs <=> rhs yields std::strong_­ordering::equal; if lhs and rhs compare unequal, lhs <=> rhs yields std::strong_­ordering::less if lhs.target->before(*rhs.target) is true, otherwise yields std::strong_ordering::greater. [Note: A result of std::strong_ordering::greater happens only in the case where rhs.target->before(*lhs.target) is true. — end note]

Iterators

Amend the requirements table for Cpp17RandomAccessIterator (22.3.5.6) requirements:

| Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | | ---------- | ---------- | ---------- | ---------- | | a < b | contextually convertible to bool | b - a > 0 | < is a total ordering relation | | a > b | contextually convertible to bool | b < a | > is a total ordering relation opposite to <. | | a >= b | contextually convertible to bool | !(a < b) | | | a <= b | contextually convertible to bool | !(a > b) | | | a <=> b | std::strong_ordering | | <=> is a total ordering relation |

chrono::duration

26.5.6 Comparisons [time.duration.comparisons] 1 In the function descriptions that follow, CT represents common_type_t<A, B>, where A and B are the types of the two arguments to the function.

template constexpr bool operator==(const duration& lhs, const duration& rhs); 2 Returns: CT(lhs).count() == CT(rhs).count().

template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator!=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

3 Returns: !(lhs == rhs).

template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator<(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

4 Returns: CT(lhs).count() < CT(rhs).count().

template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator>(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

5 Returns: rhs < lhs.

template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator<=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

6 Returns: !(rhs < lhs).

template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator>=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

7 Returns: !(lhs < rhs). For two values lhs and rhs, each of which are instances of the class template std::chrono::duration (with possibly different Rep and Period template parameters), the expression lhs <=> rhs is equivalent to CT(lhs).count() <=> CT(rhs).count(). [Note: This implies the operation is constexpr if both lhs and rhs are constexpr. — end note]

chrono::time_point

26.6.6 Comparisons [time.point.comparisons]

template constexpr bool operator==(const timepoint& lhs, const timepoint& rhs); 1 Returns: lhs.time_since_epoch() == rhs.time_since_epoch().

template<class Clock, class Duration1, class Duration2>
constexpr bool operator!=(const time_point<Clock, Duration1>& lhs, const time_point<Clock, Duration2>& rhs);

2 Returns: !(lhs == rhs).

template<class Clock, class Duration1, class Duration2>
constexpr bool operator<(const time_point<Clock, Duration1>& lhs, const time_point<Clock, Duration2>& rhs);

3 Returns: lhs.time_since_epoch() < rhs.time_since_epoch().

template<class Clock, class Duration1, class Duration2>
constexpr bool operator>(const time_point<Clock, Duration1>& lhs, const time_point<Clock, Duration2>& rhs);

4 Returns: rhs < lhs. template constexpr bool operator<=(const timepoint& lhs, const timepoint& rhs); 5 Returns: !(rhs < lhs).

template<class Clock, class Duration1, class Duration2>
constexpr bool operator>=(const time_point<Clock, Duration1>& lhs, const time_point<Clock, Duration2>& rhs);

6 Returns: !(lhs < rhs). For two values lhs and rhs, each of which are instances of the class template std::chrono::time_point (with possibly the same Clock template parameter but possibly different Duration template parameters), the expression lhs <=> rhs is equivalent to CT(lhs).time_since_epoch() <=> CT(rhs).time_since_epoch(). [Note: This implies the operation is constexpr if both lhs and rhs are constexpr. — end note]

filesystem::path

Modify the class synopsis (28.11.7 Class path [fs.class.path]) to remove the specification of comparison operators as friend functions:

namespace std::filesystem {
    class path {
    public:
        [...]

// 28.11.7.7, non-member operators friend bool operator==(const path& lhs, const path& rhs) noexcept; friend bool operator!=(const path& lhs, const path& rhs) noexcept; friend bool operator< (const path& lhs, const path& rhs) noexcept; friend bool operator<=(const path& lhs, const path& rhs) noexcept; friend bool operator> (const path& lhs, const path& rhs) noexcept; friend bool operator>=(const path& lhs, const path& rhs) noexcept; friend path operator/ (const path& lhs, const path& rhs); [...] }; }

Add the following at the end of 28.11.7.4.8 [fs.path.compare]:

For two values lhs and rhs of type std::filesystem::path, the expression lhs <=> rhs is equivalent to lhs.compare(rhs) <=> 0

28.11.7.7 Non-member functions [fs.path.nonmember]

void swap(path& lhs, path& rhs) noexcept;

1 Effects: Equivalent to lhs.swap(rhs).

size_t hash_value (const path& p) noexcept;

2 Returns: A hash value for the path p. If for two paths, p1 == p2 then hash_value(p1) == hash_value(p2).

friend bool operator==(const path& lhs, const path& rhs) noexcept;

3 Returns: !(lhs < rhs) && !(rhs < lhs).

4 [Note: Path equality and path equivalence have different semantics. — (4.1) Equality is determined by the path non-member operator==, which considers the two paths’ lexical representations only. [Example: path("foo") == "bar" is never true. — end example] — (4.2) Equivalence is determined by the equivalent() non-member function, which determines if two paths resolve (28.11.7) to the same file system entity. [Example: equivalent("foo", "bar") will be true when both paths resolve to the same file. — end example]

Programmers wishing to determine if two paths are “the same” must decide if “the same” means “the same representation” or “resolve to the same actual file”, and choose the appropriate function accordingly. — end note]

friend bool operator!=(const path& lhs, const path& rhs) noexcept;

5 Returns: !(lhs == rhs).

friend bool operator< (const path& lhs, const path& rhs) noexcept;

6 Returns: lhs.compare(rhs) < 0.

friend bool operator<=(const path& lhs, const path& rhs) noexcept;

7 Returns: !(rhs < lhs).

friend bool operator> (const path& lhs, const path& rhs) noexcept;

8 Returns: rhs < lhs.

friend bool operator>=(const path& lhs, const path& rhs) noexcept;

9 Returns: !(lhs < rhs).

friend path operator/ (const path& lhs, const path& rhs);

103 Effects: Equivalent to: return path(lhs) /= rhs;

filesystem::directory_entry

Amend the class synopsis:

namespace std::filesystem {
    class directory_entry {
    public:
        [...]

bool operator==(const directoryentry& rhs) const noexcept; bool operator!=(const directoryentry& rhs) const noexcept; bool operator< (const directoryentry& rhs) const noexcept; bool operator> (const directoryentry& rhs) const noexcept; bool operator<=(const directoryentry& rhs) const noexcept; bool operator>=(const directoryentry& rhs) const noexcept;

        [...]
    };
}

Amend 28.11.11.3:

bool operator==(const directory_entry& rhs) const noexcept;

31 Returns: pathobject == rhs.pathobject.

bool operator!=(const directory_entry& rhs) const noexcept;

32 Returns: pathobject != rhs.pathobject.

bool operator< (const directory_entry& rhs) const noexcept;

33 Returns: pathobject < rhs.pathobject.

bool operator> (const directory_entry& rhs) const noexcept;

34 Returns: pathobject > rhs.pathobject.

bool operator<=(const directory_entry& rhs) const noexcept;

35 Returns: pathobject <= rhs.pathobject.

bool operator>=(const directory_entry& rhs) const noexcept;

36 Returns: pathobject >= rhs.pathobject.

28.11.11.4 Comparisons [fs.dir.entry.comparisons]

For two values lhs and rhs of type std::filesystem::directory_entry, the expression lhs <=> rhs is equivalent to lhs.pathobject <=> rhs.pathobject.

thread::id

31.3.2.1 Class thread::id [thread.thread.id]

namespace std {
    class thread::id {
    public:
        id() noexcept;
    };

bool operator==(thread::id x, thread::id y) noexcept; bool operator!=(thread::id x, thread::id y) noexcept; bool operator<(thread::id x, thread::id y) noexcept; bool operator>(thread::id x, thread::id y) noexcept; bool operator<=(thread::id x, thread::id y) noexcept; bool operator>=(thread::id x, thread::id y) noexcept; template basicostream& operator<<(basicostream& out, thread::id id); // hash support template struct hash; template<> struct hash; }

1 An object of type thread::id provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution (31.3.2). Each thread of execution has an associated thread::id object that is not equal to the thread::id object of any other thread of execution and that is not equal to the thread::id object of any thread object that does not represent threads of execution.

2 thread::id is a trivially copyable class (10.1). The library may reuse the value of a thread::id of a terminated thread that can no longer be joined.

3 [Note: Relational operators allow thread::id objects to be used as keys in associative containers. — end note]

id() noexcept;

4 Effects: Constructs an object of type id.

5 Ensures: The constructed object does not represent a thread of execution.

bool operator==(thread::id x, thread::id y) noexcept;

6 Returns: true only if x and y represent the same thread of execution or neither x nor y represents a thread of execution.

bool operator!=(thread::id x, thread::id y) noexcept;

7 Returns: !(x == y)

bool operator<(thread::id x, thread::id y) noexcept;

8 Returns: A value such that operator< is a total ordering as described in 24.7.

bool operator>(thread::id x, thread::id y) noexcept;

9 Returns: y < x.

bool operator<=(thread::id x, thread::id y) noexcept;

10 Returns: !(y < x).

bool operator>=(thread::id x, thread::id y) noexcept;

11 Returns: !(x < y).

template<class charT, class traits>
basic_ostream<charT, traits>&
operator<< (basic_ostream<charT, traits>& out, thread::id id);

126 Effects: Inserts an unspecified text representation of id into out. For two objects of type thread::id x and y, if x == y the thread::id objects have the same text representation and if x != y the thread::id objects have distinct text representations.

137 Returns: out.

template<> struct hash<thread::id>;

148 The specialization is enabled (19.14.18).

For two values lhs and rhs of type std::thread::id, the expression lhs <=> rhs is of type std::strong_­ordering. If lhs and rhs represent the same thread of execution or neither x nor y represents a thread of execution, lhs <=> rhs yields std::strong_­ordering::equal; if lhs and rhs compare unequal, lhs <=> rhs yields a value consistent with a total order as described in 24.7.