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// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include <mutex>
#include <condition_variable>
namespace vespalib {
/**
* A latch acts like a blocking queue where the maximum capacity is a
* single element. It enables directional exchange of data where reads
* and writes are alternating.
**/
template <typename T>
class Latch {
private:
std::mutex _lock;
std::condition_variable _cond;
char _space[sizeof(T)];
bool _has_value;
void *as_void() { return &_space[0]; }
T *as_value() { return (T*)as_void(); }
public:
Latch() : _lock(), _cond(), _space(), _has_value(false) {}
~Latch() {
if (_has_value) {
as_value()->~T();
}
}
bool has_value() {
std::lock_guard guard(_lock);
return _has_value;
}
T read() {
std::unique_lock guard(_lock);
while (!_has_value) {
_cond.wait(guard);
}
T value = std::move(*as_value());
as_value()->~T();
_has_value = false;
_cond.notify_all();
return value;
}
void write(T value) {
std::unique_lock guard(_lock);
while (_has_value) {
_cond.wait(guard);
}
new (as_void()) T(std::move(value));
_has_value = true;
_cond.notify_all();
}
};
} // namespace vespalib
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