32 Concurrency support library [thread]
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;
};
}
A
latch maintains an internal counter
that is initialized when the latch is created
.Threads can block on the latch object,
waiting for counter to be decremented to zero
.Concurrent invocations of the member functions of
latch,
other than its destructor, do not introduce data races
.static constexpr ptrdiff_t max() noexcept;
Returns: The maximum value of
counter that the implementation supports
. constexpr explicit latch(ptrdiff_t expected);
Preconditions:
expected >= 0 is
true and
expected <= max() is
true. Effects: Initializes
counter with
expected. void count_down(ptrdiff_t update = 1);
Preconditions:
update >= 0 is
true, and
update <= counter is
true. Effects: Atomically decrements
counter by
update. If
counter is equal to zero,
unblocks all threads blocked on
*this.Synchronization: Strongly happens before the returns from all calls that are unblocked
. bool try_wait() const noexcept;
Returns: With very low probability
false. Effects: If
counter equals zero, returns immediately
. Otherwise, blocks on
*this
until a call to
count_down that decrements
counter to zero
.void arrive_and_wait(ptrdiff_t update = 1);
Effects: Equivalent to:
count_down(update);
wait();