std::filesystem::resize_file

Since C++17

Header: <filesystem>

Changes the size of the regular file named by p as if by POSIX truncate: if the file size was previously larger than new_size, the remainder of the file is discarded. If the file was previously smaller than new_size, the file size is increased and the new area appears as if zero-filled.

# Declarations

void resize_file( const std::filesystem::path& p,
std::uintmax_t new_size );

(since C++17)

void resize_file( const std::filesystem::path& p,
std::uintmax_t new_size,
std::error_code& ec ) noexcept;

(since C++17)

# Parameters

p: path to resize
new_size: size that the file will now have
ec: out-parameter for error reporting in the non-throwing overload

# Return value

(none)

# Notes

On systems that support sparse files, increasing the file size does not increase the space it occupies on the file system: space allocation takes place only when non-zero bytes are written to the file.

# Example

#include <filesystem>
#include <fstream>
#include <iostream>
#include <locale>
 
int main()
{
    auto p = std::filesystem::temp_directory_path() / "example.bin";
    std::ofstream{p}.put('a');
    std::cout.imbue(std::locale{"en_US.UTF8"});
    std::cout << "File size:  " << std::filesystem::file_size(p) << '\n'
              << "Free space: " << std::filesystem::space(p).free << '\n';
    std::filesystem::resize_file(p, 64*1024); // resize to 64 KB
    std::cout << "File size:  " << std::filesystem::file_size(p) << '\n'
              << "Free space: " << std::filesystem::space(p).free << '\n';
    std::filesystem::remove(p);
}