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
std::filesystem::path& p);

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

std::error_code& ec) noexcept;
void last_write_time(const std::filesystem::path& p,

std::filesystem::file_time_type new_time);
void last_write_time(const std::filesystem::path& p, (2) (since C++17)
std::filesystem::file_time_type new_time,

std::error_code& ec) noexcept;

1) 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.
2) 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) The time of the last modification of p
2) (none)

Exceptions

The overload that does not take a std::error_code& parameter throws
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. The overload
taking a std::error_code& parameter sets it to the OS API error code if an OS API
call fails, and executes ec.clear() if no errors occur. Any overload not marked
noexcept may throw std::bad_alloc if memory allocation fails.

Notes

It is not guaranteed that immediately after setting the write time, the value
returned by (1) is the same as what was passed as the argument to (2) 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);

std::filesystem::last_write_time(p, ftime + 1h); // move file write time 1 hour to the future
ftime = std::filesystem::last_write_time(p); // read back from the filesystem
std::cout << std::format("File write time is {}\n", ftime);

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

Possible output:

File write time is Sun May 9 23:29:58 2021
File write time is Mon May 10 00:29:58 2021