tmp/tmpwwyjojy0/{from.md → to.md}
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| 1 |
+
#### Class `latch` <a id="thread.latch.class">[[thread.latch.class]]</a>
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``` cpp
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namespace std {
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class latch {
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public:
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static constexpr ptrdiff_t max() noexcept;
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| 8 |
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constexpr explicit latch(ptrdiff_t expected);
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+
~latch();
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latch(const latch&) = delete;
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latch& operator=(const latch&) = delete;
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void count_down(ptrdiff_t update = 1);
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bool try_wait() const noexcept;
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void wait() const;
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void arrive_and_wait(ptrdiff_t update = 1);
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private:
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ptrdiff_t counter; // exposition only
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};
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}
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```
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A `latch` maintains an internal counter that is initialized when the
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latch is created. Threads can block on the latch object, waiting for
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counter to be decremented to zero.
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+
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Concurrent invocations of the member functions of `latch`, other than
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| 31 |
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its destructor, do not introduce data races.
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``` cpp
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static constexpr ptrdiff_t max() noexcept;
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```
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*Returns:* The maximum value of `counter` that the implementation
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supports.
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``` cpp
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constexpr explicit latch(ptrdiff_t expected);
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```
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+
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*Preconditions:* `expected >= 0` is `true` and `expected <= max()` is
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| 45 |
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`true`.
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*Effects:* Initializes `counter` with `expected`.
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*Throws:* Nothing.
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| 50 |
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``` cpp
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void count_down(ptrdiff_t update = 1);
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```
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*Preconditions:* `update >= 0` is `true`, and `update <= counter` is
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| 56 |
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`true`.
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*Effects:* Atomically decrements `counter` by `update`. If `counter` is
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| 59 |
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equal to zero, unblocks all threads blocked on `*this`.
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*Synchronization:* Strongly happens before the returns from all calls
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| 62 |
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that are unblocked.
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| 63 |
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*Throws:* `system_error` when an exception is
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| 65 |
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required [[thread.req.exception]].
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| 66 |
+
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*Error conditions:* Any of the error conditions allowed for mutex
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| 68 |
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types [[thread.mutex.requirements.mutex]].
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| 69 |
+
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``` cpp
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| 71 |
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bool try_wait() const noexcept;
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| 72 |
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```
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*Returns:* With very low probability `false`. Otherwise `counter == 0`.
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+
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``` cpp
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| 77 |
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void wait() const;
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| 78 |
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```
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| 79 |
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*Effects:* If `counter` equals zero, returns immediately. Otherwise,
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| 81 |
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blocks on `*this` until a call to `count_down` that decrements `counter`
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to zero.
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*Throws:* `system_error` when an exception is
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| 85 |
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required [[thread.req.exception]].
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| 86 |
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*Error conditions:* Any of the error conditions allowed for mutex
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| 88 |
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types [[thread.mutex.requirements.mutex]].
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| 89 |
+
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| 90 |
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``` cpp
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| 91 |
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void arrive_and_wait(ptrdiff_t update = 1);
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| 92 |
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```
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| 93 |
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| 94 |
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*Effects:* Equivalent to:
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| 95 |
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``` cpp
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+
count_down(update);
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| 98 |
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wait();
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| 99 |
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```
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