std::filesystem::last_write_time

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 std::filesystem::path& p, std::error_code& ec ) noexcept;	(1)	(since C++17)
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, std::filesystem::file_time_type new_time, std::error_code& ec ) noexcept;	(2)	(since C++17)

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 <fstream>
#include <iostream>
#include <print>
 
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::println("File write time is {}", 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::println("File write time is {}", 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

Compiler support
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Feature test macros (C++20)
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Technical specifications
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External libraries

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