32 Concurrency support library [thread]

32.4 Threads [thread.threads]

32.4.3 Class thread [thread.thread.class]

32.4.3.2 Class thread​::​id [thread.thread.id]

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namespace std { class thread::id { public: id() noexcept; }; bool operator==(thread::id x, thread::id y) noexcept; strong_ordering operator<=>(thread::id x, thread::id y) noexcept; template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, thread::id id); template<class charT> struct formatter<thread::id, charT>; // hash support template<class T> struct hash; template<> struct hash<thread::id>; }
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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 ([thread.thread.class]).
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.
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The text representation for the character type charT of an object of type thread​::​id is an unspecified sequence of charT such that, for two objects of type thread​::​id x and y, if x == y is true, the thread​::​id objects have the same text representation, and if x != y is true, the thread​::​id objects have distinct text representations.
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thread​::​id is a trivially copyable class ([class.prop]).
The library may reuse the value of a thread​::​id of a terminated thread that can no longer be joined.
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[Note 1: 
Relational operators allow thread​::​id objects to be used as keys in associative containers.
— end note]
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id() noexcept;
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Postconditions: The constructed object does not represent a thread of execution.
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bool operator==(thread::id x, thread::id y) noexcept;
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Returns: true only if x and y represent the same thread of execution or neither x nor y represents a thread of execution.
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strong_ordering operator<=>(thread::id x, thread::id y) noexcept;
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Let P(x, y) be an unspecified total ordering over thread​::​id as described in [alg.sorting].
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Returns: strong_ordering​::​less if P(x, y) is true.
Otherwise, strong_ordering​::​greater if P(y, x) is true.
Otherwise, strong_ordering​::​equal.
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, thread::id id);
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Effects: Inserts the text representation for charT of id into out.
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Returns: out.
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template<class charT> struct formatter<thread::id, charT>;
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formatter<thread​::​id, charT> interprets format-spec as a thread-id-format-spec.
The syntax of format specifications is as follows:
thread-id-format-spec :
fill-and-align width
[Note 2: 
The productions fill-and-align and width are described in [format.string.std].
— end note]
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If the align option is omitted it defaults to >.
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A thread​::​id object is formatted by writing its text representation for charT to the output with additional padding and adjustments as specified by the format specifiers.
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template<> struct hash<thread::id>;
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The specialization is enabled ([unord.hash]).