NAME¶

std::filesystem::last_write_time - std::filesystem::last_write_time

Synopsis¶

Defined in header <filesystem>
std::filesystem::file_time_type last_write_time( const (1) (since C++17)
std::filesystem::path& p );
std::filesystem::file_time_type last_write_time( const
std::filesystem::path& p, (2) (since C++17)
std::error_code&
ec ) noexcept;
void last_write_time( const std::filesystem::path& p, (3) (since C++17)
std::filesystem::file_time_type new_time );
void last_write_time( const std::filesystem::path& p,

std::filesystem::file_time_type new_time, (4) (since C++17)

std::error_code& ec ) noexcept;

1,2) Returns 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) Changes 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.

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)
gets the time of the last data modification of the file to which the
last_write_time directory entry refers
(public member function of std::filesystem::directory_entry)