第三节 · 逻辑的空间 · 定长状态表bitset

C++11 的 <chrono> 库提供了类型安全的时间处理功能，C++20 进一步增强了日历和时区支持。

---

时间点和时长

#include <chrono>
using namespace std::chrono;

int main()
{
    // 获取当前时间点
    auto now = system_clock::now();
    
    // 时长
    seconds sec(5);
    milliseconds ms(5000);
    microseconds us(5000000);
    
    // 时长转换
    cout << duration_cast<seconds>(ms).count() << "s" << endl;  // 5s
    cout << duration_cast<milliseconds>(sec).count() << "ms" << endl;  // 5000ms
    
    return 0;
}

---

测量时间

#include <chrono>
using namespace std::chrono;

void doWork()
{
    // 模拟耗时操作
    for (int i = 0; i < 1000000; ++i)
    {
        volatile int x = i * i;
    }
}

int main()
{
    auto start = high_resolution_clock::now();
    
    doWork();
    
    auto end = high_resolution_clock::now();
    auto duration = duration_cast<microseconds>(end - start);
    
    cout << "Time: " << duration.count() << " us" << endl;
    
    // 更精确的输出
    cout << "Time: " << duration.count() / 1000.0 << " ms" << endl;
    
    return 0;
}

---

不同的时钟

时钟	说明
system_clock	系统时间，可以与日历时间转换
steady_clock	单调时钟，适合测量时间间隔
high_resolution_clock	最高精度时钟
utc_clock	UTC 时间（C++20）

int main()
{
    // system_clock 可以转换为 time_t
    auto now = system_clock::now();
    time_t t = system_clock::to_time_t(now);
    cout << "Current time: " << ctime(&t);
    
    // steady_clock 用于测量间隔
    auto t1 = steady_clock::now();
    // ... 工作 ...
    auto t2 = steady_clock::now();
    auto elapsed = t2 - t1;
    
    return 0;
}

---

时长字面量（C++14）

using namespace std::chrono_literals;

int main()
{
    auto hours = 2h;
    auto minutes = 30min;
    auto seconds = 45s;
    auto millis = 100ms;
    auto micros = 500us;
    auto nanos = 1000ns;
    
    // 组合
    auto duration = 1h + 30min + 45s;
    cout << duration_cast<seconds>(duration).count() << "s" << endl;  // 5445s
    
    return 0;
}

---

定时器实现

#include <chrono>
#include <thread>

class Timer
{
    using Clock = steady_clock;
    Clock::time_point start;
    
public:
    Timer() : start(Clock::now()) {}
    
    void reset()
    {
        start = Clock::now();
    }
    
    double elapsed() const
    {
        auto now = Clock::now();
        return duration_cast<milliseconds>(now - start).count() / 1000.0;
    }
};

int main()
{
    Timer timer;
    
    this_thread::sleep_for(500ms);
    cout << "Elapsed: " << timer.elapsed() << "s" << endl;
    
    timer.reset();
    this_thread::sleep_for(200ms);
    cout << "Elapsed: " << timer.elapsed() << "s" << endl;
    
    return 0;
}

---

日期处理（C++20）

#include <chrono>
using namespace std::chrono;

int main()
{
    // 创建日期
    year_month_day today = 2024y/December/25d;
    
    // 也可以用整数
    year_month_day date{year{2024}, month{12}, day{25}};
    
    // 检查有效性
    if (today.ok())
    {
        cout << "Valid date" << endl;
    }
    
    // 访问组件
    cout << "Year: " << int(today.year()) << endl;
    cout << "Month: " << unsigned(today.month()) << endl;
    cout << "Day: " << unsigned(today.day()) << endl;
    
    // 星期几
    weekday wd{sys_days{today}};
    cout << "Weekday: " << wd << endl;  // Wed
    
    return 0;
}

---

日期算术（C++20）

#include <chrono>
using namespace std::chrono;

int main()
{
    auto today = 2024y/December/1d;
    
    // 加减天数
    auto tomorrow = sys_days{today} + days{1};
    
    // 加减月份
    auto next_month = today + months{1};
    
    // 加减年份
    auto next_year = today + years{1};
    
    // 计算两个日期之间的天数
    auto d1 = 2024y/January/1d;
    auto d2 = 2024y/December/31d;
    auto diff = sys_days{d2} - sys_days{d1};
    cout << "Days in 2024: " << diff.count() << endl;  // 365 or 366
    
    return 0;
}

---

时区（C++20）

#include <chrono>
using namespace std::chrono;

int main()
{
    // 获取当前 UTC 时间
    auto utc_now = utc_clock::now();
    
    // 获取时区
    auto tz = locate_zone("Asia/Shanghai");
    auto local = tz->to_local(system_clock::now());
    
    // 输出
    cout << "Local time: " << local << endl;
    
    // 列出所有时区
    for (const auto& tz : get_tzdb().zones)
    {
        // cout << tz.name() << endl;
    }
    
    return 0;
}

---

格式化时间（C++20）

#include <chrono>
#include <format>
using namespace std::chrono;

int main()
{
    auto now = system_clock::now();
    
    // 使用 format
    cout << format("{:%Y-%m-%d %H:%M:%S}", now) << endl;
    cout << format("{:%A, %B %d, %Y}", now) << endl;
    
    // 自定义格式
    cout << format("{:%Y年%m月%d日}", now) << endl;
    
    return 0;
}

---

实用函数

// 计时器包装函数
template<typename F, typename... Args>
auto measureTime(F&& func, Args&&... args)
{
    auto start = high_resolution_clock::now();
    forward<F>(func)(forward<Args>(args)...);
    auto end = high_resolution_clock::now();
    return duration_cast<milliseconds>(end - start);
}

// 延迟执行
void delayed(milliseconds delay, function<void()> task)
{
    this_thread::sleep_for(delay);
    task();
}

int main()
{
    auto duration = measureTime([]
    {
        for (int i = 0; i < 1000000; ++i) {}
    });
    cout << "Execution time: " << duration.count() << "ms" << endl;
    
    delayed(1000ms, []
    {
        cout << "Delayed message!" << endl;
    });
    
    return 0;
}

---

习题

• 实现一个秒表类，支持开始、暂停、继续、重置功能。

• 计算任意两个日期之间相差的天数。

• 编写一个函数，输出距离指定日期还有多少天。