std::filesy..._write_time(3)

💅 std::filesy..._write_time(3) — the C++ Standard Libary manual, bestie! 💅

Manual name: std::filesy..._write_time(3). Yes, the title is literally the same on both sides of the header, and we are moving on. Iconic behavior only. ✨

🌸 NAME 🌸

std::filesystem::last_write_time — and yep, the NAME line is literally std::filesystem::last_write_time - std::filesystem::last_write_time. No notes. 💅

📜 Synopsis 📜

Defined in header <filesystem>.

All four overloads are since C++17, periodt. 🫶

(1) (since C++17)
std::filesystem::file_time_type last_write_time( const std::filesystem::path& p );

(2) (since C++17)
std::filesystem::file_time_type last_write_time( const std::filesystem::path& p,
                                                 std::error_code& ec ) noexcept;

(3) (since C++17)
void last_write_time( const std::filesystem::path& p,
                      std::filesystem::file_time_type new_time );

(4) (since C++17)
void last_write_time( const std::filesystem::path& p,
                      std::filesystem::file_time_type new_time,
                      std::error_code& ec ) noexcept;

Okay so bestie, here's the tea: 🍓

• 1,2) These return the time of the last modification of p, determined as if by accessing the member st_mtime of the POSIX stat (symlinks are followed). The non-throwing overload returns file_time_type::min() on errors.
• 3,4) These change the time of the last modification of p, as if by POSIX futimens (symlinks are followed).

🧚 Parameters 🧚

• p — path to examine or modify
• new_time — new modification time
• ec — out-parameter for error reporting in the non-throwing overload

👑 Return value 👑

• 1,2) The time of the last modification of p.
• 3,4) (none)

💀 Exceptions 💀

Any overload not marked noexcept may throw std::bad_alloc if memory allocation fails.

• 1,3) Throws std::filesystem::filesystem_error on underlying OS API errors, constructed with p as the first path argument and the OS error code as the error code argument.
• 2,4) Sets a std::error_code& parameter to the OS API error code if an OS API call fails, and executes ec.clear() if no errors occur.

🦋 Notes 🦋

It is not guaranteed that immediately after setting the write time, the value returned by (1,2) is the same as what was passed as the argument to (3,4) because the file system's time may be more granular than filesystem::file_time_type. Literally the filesystem said "best I can do is this much precision." 😭

🎬 Example 🎬

// Run this code

#include <chrono>
#include <filesystem>
#include <format>
#include <fstream>
#include <iostream>

using namespace std::chrono_literals;

int main()
{
    auto p = std::filesystem::temp_directory_path() / "example.bin";
    std::ofstream{p.c_str()}.put('a'); // create file

    std::filesystem::file_time_type ftime = std::filesystem::last_write_time(p);
    std::cout << std::format("File write time is {}\n", ftime);

    // move file write time 1 hour to the future
    std::filesystem::last_write_time(p, ftime + 1h);

    // read back from the filesystem
    ftime = std::filesystem::last_write_time(p);
    std::cout << std::format("File write time is {}\n", ftime);

    std::filesystem::remove(p);
}

✨ Possible output ✨

File write time is 2023-09-04 19:33:24.702639224
File write time is 2023-09-04 20:33:24.702639224

🔥 See also 🔥

• file_time_type — represents file time values. (C++17) (typedef)
• last_write_time — gets the time of the last data modification of the file to which the last_write_time directory entry refers. (C++17) (public member function of std::filesystem::directory_entry)

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That's the whole page, bestie 💖 Source/version: http://cppreference.com — date: 2024.06.10 — manual: std::filesy..._write_time(3). Byeee, go touch some filesystem timestamps 🫶✨