std::filesystem::last_write_time

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Filesystem library

Classes
filesystem::path filesystem::filesystem_error filesystem::directory_entry filesystem::directory_iterator filesystem::recursive_directory_iterator      filesystem::file_status	filesystem::space_info filesystem::file_type filesystem::perms filesystem::perm_options filesystem::copy_options filesystem::directory_options filesystem::file_time_type
Functions
filesystem::absolute filesystem::canonicalfilesystem::weakly_canonical filesystem::relativefilesystem::proximate filesystem::copy filesystem::copy_file filesystem::copy_symlink filesystem::create_directory filesystem::create_directories filesystem::create_hard_link filesystem::create_symlink filesystem::create_directory_symlink filesystem::current_path	filesystem::exists filesystem::equivalent filesystem::file_size filesystem::hard_link_count filesystem::last_write_time filesystem::permissions filesystem::read_symlink filesystem::remove filesystem::remove_all filesystem::rename filesystem::resize_file filesystem::space filesystem::status filesystem::symlink_status filesystem::temp_directory_path
File types
filesystem::is_block_file filesystem::is_character_file filesystem::is_directory filesystem::is_empty filesystem::status_known	filesystem::is_fifo filesystem::is_other filesystem::is_regular_file filesystem::is_socket filesystem::is_symlink

 

Defined in header <filesystem>
std::filesystem::file_time_type last_write_time( const std::filesystem::path& p );	(1)	(since C++17)
std::filesystem::file_time_type last_write_time( const std::filesystem::path& p,                                                  std::error_code& ec ) noexcept;	(2)	(since C++17)
void last_write_time( const std::filesystem::path& p,                       std::filesystem::file_time_type new_time );	(3)	(since C++17)
void last_write_time( const std::filesystem::path& p,                       std::filesystem::file_time_type new_time,                       std::error_code& ec ) noexcept;	(4)	(since C++17)

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