# std::filesystem::last_write_time

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< cpp‎ | filesystem

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`)

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