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cppdraft/thread/latch/class.md

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+[thread.latch.class]
+
+# 32 Concurrency support library [[thread]](./#thread)
+
+## 32.9 Coordination types [[thread.coord]](thread.coord#thread.latch.class)
+
+### 32.9.2 Latches [[thread.latch]](thread.latch#class)
+
+#### 32.9.2.3 Class latch [thread.latch.class]
+
+namespace std {class latch {public:static constexpr ptrdiff_t max() noexcept; constexpr explicit latch(ptrdiff_t expected); ~latch();
+
+ latch(const latch&) = delete;
+ latch& operator=(const latch&) = delete; void count_down(ptrdiff_t update = 1); bool try_wait() const noexcept; void wait() const; void arrive_and_wait(ptrdiff_t update = 1); private: ptrdiff_t counter; // *exposition only*};}
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10720)
+
+A latch maintains an internal counter
+that is initialized when the latch is created[.](#1.sentence-1)
+
+Threads can block on the latch object,
+waiting for counter to be decremented to zero[.](#1.sentence-2)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10726)
+
+Concurrent invocations of the member functions of latch,
+other than its destructor, do not introduce data races[.](#2.sentence-1)
+
+[🔗](#lib:max,latch)
+
+`static constexpr ptrdiff_t max() noexcept;
+`
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10736)
+
+*Returns*: The maximum value of counter that the implementation supports[.](#3.sentence-1)
+
+[🔗](#lib:latch,constructor)
+
+`constexpr explicit latch(ptrdiff_t expected);
+`
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10747)
+
+*Preconditions*: expected >= 0 is true andexpected <= max() is true[.](#4.sentence-1)
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10752)
+
+*Effects*: Initializes counter with expected[.](#5.sentence-1)
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10756)
+
+*Throws*: Nothing[.](#6.sentence-1)
+
+[🔗](#lib:count_down,latch)
+
+`void count_down(ptrdiff_t update = 1);
+`
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10767)
+
+*Preconditions*: update >= 0 is true, andupdate <= counter is true[.](#7.sentence-1)
+
+[8](#8)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10772)
+
+*Effects*: Atomically decrements counter by update[.](#8.sentence-1)
+
+If counter is equal to zero,
+unblocks all threads blocked on *this[.](#8.sentence-2)
+
+[9](#9)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10778)
+
+*Synchronization*: Strongly happens before the returns from all calls that are unblocked[.](#9.sentence-1)
+
+[10](#10)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10782)
+
+*Throws*: system_error when an exception is required ([[thread.req.exception]](thread.req.exception "32.2.2 Exceptions"))[.](#10.sentence-1)
+
+[11](#11)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10786)
+
+*Error conditions*: Any of the error conditions
+allowed for mutex types ([[thread.mutex.requirements.mutex]](thread.mutex.requirements.mutex "32.6.4.2 Mutex types"))[.](#11.sentence-1)
+
+[🔗](#lib:try_wait,latch)
+
+`bool try_wait() const noexcept;
+`
+
+[12](#12)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10798)
+
+*Returns*: With very low probability false[.](#12.sentence-1)
+
+Otherwise counter == 0[.](#12.sentence-2)
+
+[🔗](#lib:wait,latch)
+
+`void wait() const;
+`
+
+[13](#13)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10809)
+
+*Effects*: If counter equals zero, returns immediately[.](#13.sentence-1)
+
+Otherwise, blocks on *this until a call to count_down that decrements counter to zero[.](#13.sentence-2)
+
+[14](#14)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10816)
+
+*Throws*: system_error when an exception is required ([[thread.req.exception]](thread.req.exception "32.2.2 Exceptions"))[.](#14.sentence-1)
+
+[15](#15)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10820)
+
+*Error conditions*: Any of the error conditions
+allowed for mutex types ([[thread.mutex.requirements.mutex]](thread.mutex.requirements.mutex "32.6.4.2 Mutex types"))[.](#15.sentence-1)
+
+[🔗](#lib:arrive_and_wait,latch)
+
+`void arrive_and_wait(ptrdiff_t update = 1);
+`
+
+[16](#16)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10832)
+
+*Effects*: Equivalent to:count_down(update);
+wait();

cppdraft/thread/latch/general.md

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+[thread.latch.general]
+
+# 32 Concurrency support library [[thread]](./#thread)
+
+## 32.9 Coordination types [[thread.coord]](thread.coord#thread.latch.general)
+
+### 32.9.2 Latches [[thread.latch]](thread.latch#general)
+
+#### 32.9.2.1 General [thread.latch.general]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10677)
+
+A latch is a thread coordination mechanism
+that allows any number of threads to block
+until an expected number of threads arrive at the latch
+(via the count_down function)[.](#1.sentence-1)
+
+The expected count is set when the latch is created[.](#1.sentence-2)
+
+An individual latch is a single-use object;
+once the expected count has been reached, the latch cannot be reused[.](#1.sentence-3)