C++20新特性的小细节
之前我整理过一篇C++20新特性的文章全网首发!!C++20新特性全在这一张图里了,里面提到过latch、barrier和semaphore,但是没有详细介绍过三者的作用和区别,这里详细介绍下。
latch
这个可能大多数人都有所了解,这就是我们经常会用到的CountDownLatch。用于使一个线程先阻塞,等待其他线程完成各自的工作后再继续执行。
CountDownLatch是通过计数器实现,计数器的初始值为线程的数量。每当一个线程完成了自己的任务后,计数器的值就会减1。当计数器值到达0时,它表示所有的线程已经完成了任务,然后等待的线程就可以打断阻塞去继续执行任务。
自己之前实现过一个CountDownLatch,源码大概这样:
CountDownLatch::CountDownLatch(int32_t count) : count_(count) {}
void CountDownLatch::CountDown() {
std::unique_lock<std::mutex> lock(mutex_);
--count_;
if (count_ == 0) {
cv_.notify_all();
}
}
void CountDownLatch::Await(int32_t time_ms) {
std::unique_lock<std::mutex> lock(mutex_);
while (count_ > 0) {
if (time_ms > 0) {
cv_.wait_for(lock, std::chrono::milliseconds(time_ms));
} else {
cv_.wait(lock);
}
}
}
int32_t CountDownLatch::GetCount() const {
std::unique_lock<std::mutex> lock(mutex_);
return count_;
}
barrier
许多线程在阻塞点阻塞,当到达阻塞点的线程达到一定数量时,会执行完成的回调,然后解除所有相关线程的阻塞,然后重置线程计数器,继续开始下一阶段的阻塞。
假设有很多线程并发执行,并在一个循环中执行一些计算。进一步假设一旦这些计算完成,需要在线程开始其循环的新迭代之前对结果进行一些处理。
看以下示例代码(摘自cppreference):
int main() {
const auto workers = { "anil", "busara", "carl" };
auto on_completion = []() noexcept {
// locking not needed here
static auto phase = "... done\n" "Cleaning up...\n";
std::cout << phase;
phase = "... done\n";
};
std::barrier sync_point(std::ssize(workers), on_completion);
auto work = [&](std::string name) {
std::string product = " " + name + " worked\n";
std::cout << product; // ok, op<< call is atomic
sync_point.arrive_and_wait();
product = " " + name + " cleaned\n";
std::cout << product;
sync_point.arrive_and_wait();
};
std::cout << "Starting...\n";
std::vector<std::thread> threads;
for (auto const& worker : workers) {
threads.emplace_back(work, worker);
}
for (auto& thread : threads) {
thread.join();
}
}
可能的输出如下:
Starting...
anil worked
carl worked
busara worked
done
Cleaning up...
busara cleaned
carl cleaned
anil cleaned
done
semaphore
信号量,这个估计大家都很熟悉,本质也是个计数器,主要有两个方法:
acquire():递减计数器,当计数器为零时阻塞,直到计数器再次递增。
release():递增计数器(可传递具体数字),并解除在acquire调用中的线程的阻塞。
示例代码如下:
#include
#include
#include
#include
std::binary_semaphore
smphSignalMainToThread(0),
smphSignalThreadToMain(0);
void ThreadProc() {
smphSignalMainToThread.acquire();
std::cout << "[thread] Got the signal\n"; // response message
using namespace std::literals;
std::this_thread::sleep_for(3s);
std::cout << "[thread] Send the signal\n"; // message
smphSignalThreadToMain.release();
}
int main() {
std::thread thrWorker(ThreadProc);
std::cout << "[main] Send the signal\n"; // message
smphSignalMainToThread.release();
smphSignalThreadToMain.acquire();
std::cout << "[main] Got the signal\n"; // response message
thrWorker.join();
}
输出如下:
[main] Send the signal
[thread] Got the signal
[thread] Send the signal
[main] Got the signal
信号量也可以当作条件变量使用,这个我估计大家应该知道怎么做。
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