std::atomic<T>::exchange

#include <algorithm>
#include <atomic>
#include <cstddef>
#include <iostream>
#include <syncstream>
#include <thread>
#include <vector>
 
int main()
{
    constexpr int thread_count{5};
    constexpr int sum{5};
 
    std::atomic<int> atom{0};
    std::atomic<int> counter{0};
 
    auto increment_to_sum = [&](const int id)
    {
        for (int next = 0; next < sum;)
        {
            // 每个线程各自写入自身所知的值
            const int current = atom.exchange(next);
            counter++;
            // 同步写入，避免被其他线程中断
            std::osyncstream(std::cout)
                << "线程 #" << id << "（id=" << std::this_thread::get_id()
                << "）写入 " << next << "，替换旧值 "
                << current << "。\n";
            next = std::max(current, next) + 1;
        }
    };
 
    std::vector<std::thread> v;
    for (std::size_t i = 0; i < thread_count; ++i)
        v.emplace_back(increment_to_sum, i);
 
    for (auto& tr : v)
        tr.join();
 
    std::cout << thread_count << " 个线程总共用了 "
              << counter << " 次将 0 增加到 " << sum << "。\n";
}