std::asctime(3)

💖 NAME 💖

okay bestie, welcome to std::asctime(3) — the C++ Standard Library’s fixed-format date/time-to-text girlie. std::asctime is literally the function name and the whole vibe: it converts calendar time into a string. 💅

✨ Synopsis ✨

Defined in header <ctime> 🎀

char* asctime( const std::tm* time_ptr );

okay so this function converts a given calendar time std::tm to a textual representation in this fixed 25-character form: Www Mmm dd hh:mm:ss yyyy\n. 🕰️

• Www — three-letter English abbreviated day of the week from time_ptr->tm_wday, one of Mon, Tue, Wed, Thu, Fri, Sat, Sun. 🗓️
• Mmm — three-letter English abbreviated month name from time_ptr->tm_mon, one of Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec. 🌸
• dd — 2-digit day of the month from timeptr->tm_mday as if printed by sprintf using %2d. ✨
• hh — 2-digit hour from timeptr->tm_hour as if printed by sprintf using %.2d. ⏰
• mm — 2-digit minute from timeptr->tm_min as if printed by sprintf using %.2d. 🕰️
• ss — 2-digit second from timeptr->tm_sec as if printed by sprintf using %.2d. ⏱️
• yyyy — 4-digit year from timeptr->tm_year + 1900 as if printed by sprintf using %4d. 🗓️

🚨 Undefined behavior alert: The behavior is undefined if any member of *time_ptr is outside its normal range. bestie, do not play with this. 😭

🚨 Another undefined behavior alert: The behavior is undefined if the calendar year indicated by time_ptr->tm_year has more than 4 digits or is less than the year 1000. literally do not. 💀

The function does not support localization, and the newline character cannot be removed. 😭

The function modifies static storage and is not thread-safe. 🚩

🫶 Parameters 🫶

• time_ptr — pointer to a std::tm object specifying the time to print. 🎀

🎀 Return value 🎀

You get back a pointer to a static null-terminated character string holding the textual representation of date and time. ✨ The string may be shared between std::asctime and std::ctime, and may be overwritten on each invocation of any of those functions.

🌸 Notes 🌸

This function returns a pointer to static data and is not thread-safe. 🚩 POSIX marks this function obsolete and recommends locale-dependent std::strftime instead. In std::locale("C") the std::strftime format string "%c\n" will be an exact match to std::asctime output, while in other locales the format string "%a %b %e %H:%M:%S %Y\n" will be a potentially closer but not always exact match. 🦋

POSIX limits undefined behaviors only to the cases when the output string would be longer than 25 characters, when timeptr->tm_wday or timeptr->tm_mon are not within the expected ranges, or when timeptr->tm_year exceeds INT_MAX-1990. 👑

Some implementations handle timeptr->tm_mday == 0 as meaning the last day of the preceding month. 🌙

🦋 Example 🦋

// Run this code

#include <ctime>
#include <iomanip>
#include <iostream>

int main()
{
    const std::time_t now = std::time(nullptr);

    for (const char* localeName : {"C", "en_US.utf8", "de_DE.utf8", "ja_JP.utf8"})
    {
        std::cout << "locale " << localeName << ":\n" << std::left;
        std::locale::global(std::locale(localeName));

        std::cout << std::setw(40) << "    asctime" << std::asctime(std::localtime(&now));

        // strftime output for comparison:
        char buf[64];
        if (strftime(buf, sizeof buf, "%c\n", std::localtime(&now)))
            std::cout << std::setw(40) << "    strftime %c" << buf;

        if (strftime(buf, sizeof buf, "%a %b %e %H:%M:%S %Y\n", std::localtime(&now)))
            std::cout << std::setw(40) << "    strftime %a %b %e %H:%M:%S %Y" << buf;

        std::cout << '\n';
    }
}

🍓 Possible output 🍓

locale C:
    asctime                             Wed Nov  4 00:45:01 2020
    strftime %c                         Wed Nov  4 00:45:01 2020
    strftime %a %b %e %H:%M:%S %Y       Wed Nov  4 00:45:01 2020

locale en_US.utf8:
    asctime                             Wed Nov  4 00:45:01 2020
    strftime %c                         Wed 04 Nov 2020 12:45:01 AM UTC
    strftime %a %b %e %H:%M:%S %Y       Wed Nov  4 00:45:01 2020

locale de_DE.utf8:
    asctime                             Wed Nov  4 00:45:01 2020
    strftime %c                         Mi 04 Nov 2020 00:45:01 UTC
    strftime %a %b %e %H:%M:%S %Y       Mi Nov  4 00:45:01 2020

locale ja_JP.utf8:
    asctime                             Wed Nov  4 00:45:01 2020
    strftime %c                         2020年11月04日 00時45分01秒
    strftime %a %b %e %H:%M:%S %Y       水 11月  4 00:45:01 2020

🌈 See also 🌈

• ctime — converts a std::time_t object to a textual representation (function). 💖
• strftime — converts a std::tm object to custom textual representation (function). ✨
• put_time — formats and outputs a date/time value according to the specified format (C++11) (function template). 🎀
• C documentation for asctime. 🦋

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source: http://cppreference.com ✨ version/date: 2024.06.10 ✨ page: std::asctime(3) — that’s the whole manual, bestie. mwah 💋