std::chrono在C 中用于處理時(shí)間，包括獲取當(dāng)前時(shí)間、測(cè)量執(zhí)行時(shí)間、操作時(shí)間點(diǎn)與持續(xù)時(shí)間及格式化解析時(shí)間。1. 獲取當(dāng)前時(shí)間使用std::chrono::system_clock::now()，可轉(zhuǎn)換為可讀字符串但系統(tǒng)時(shí)鐘可能不單調(diào)；2. 測(cè)量執(zhí)行時(shí)間應(yīng)使用std::chrono::steady_clock以確保單調(diào)性，并通過duration_cast轉(zhuǎn)換為毫秒、秒等單位；3. 時(shí)間點(diǎn)(time_point)和持續(xù)時(shí)間(duration)可相互操作，但需注意單位兼容性和時(shí)鐘紀(jì)元(epoch)差異；4. C 20支持時(shí)間格式化與解析，需使用std::format和std::parse，依賴編譯器對(duì)C 20的支持。

Using std::chrono in C can be really useful once you get the hang of it. It’s part of the standard library and gives you solid tools for handling time — whether that’s measuring durations, working with timestamps, or dealing with time zones in C 20 and beyond.

Getting the current time

To grab the current point in time, you usually go with std::chrono::system_clock::now(). That gives you a time_point representing right now.

auto now = std::chrono::system_clock::now();

This is often used when you want to measure how long something takes or just log a timestamp. If you need to convert it to something readable like a string, you’ll typically convert it to time_t and use ctime or similar:

std::time_t now_c = std::chrono::system_clock::to_time_t(now);
std::cout << "Current time: " << std::ctime(&now_c);

Keep in mind that system_clock might not be monotonic — if someone changes the system time, it could jump forward or backward. For timing purposes, read on.

Measuring execution time accurately

When you want to measure how long a piece of code runs, steady_clock is your friend. It’s monotonic (won’t go backwards), which makes it safe for timing.

Here’s a common pattern:

auto start = std::chrono::steady_clock::now();

// ... do some work ...

auto end = std::chrono::steady_clock::now();
auto duration = end - start;

If you want to show this in milliseconds or seconds, you’ll cast it using duration_cast:

auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();
std::cout << "Took " << ms << " ms\n";

You can also use microseconds, nanoseconds, or even seconds. Just keep in mind that converting from higher precision (like nanoseconds) to lower (like seconds) will truncate unless you cast properly.

Working with time points and durations

• A time_point is a specific moment.
• A duration is a span of time (like 5 seconds).

They’re separate types, but they work together. You can add a duration to a time_point to get a new time_point:

auto then = now   std::chrono::hours(2);

This is handy when scheduling events or waiting until a certain time. Just make sure both sides of the operation are using compatible units — mixing hours and milliseconds won’t cause errors, but it might not do what you expect unless you explicitly convert.

Also, don’t assume that all clocks start at zero — their epoch (starting point) varies:

• system_clock typically starts in 1970 (like Unix time).
• steady_clock has an arbitrary epoch, so comparing its time_points across runs doesn’t make sense.

Formatting and parsing time (C 20 )

With C 20, <chrono> got better support for formatting dates and times directly:

auto now = std::chrono::system_clock::now();
std::cout << "Formatted: " << std::format("{:%Y-%m-%d %H:%M}", now) << "\n";

Parsing time strings also became possible:

std::istringstream ss("2024-03-15 12:30");
std::chrono::system_clock::time_point tp;
ss >> std::parse("%Y-%m-%d %H:%M", tp);

This is super helpful when reading logs or config files with timestamps. But remember, these features require C 20 and good compiler support (like GCC 13 , Clang 15 , or MSVC with latest STL).

So yeah, std::chrono is pretty powerful once you understand the basic types and when to use each clock. Not too hard, just a bit easy to mix up at first.

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