std::filesystem::resize_file (3) - Linux Manuals
Defined in header <filesystem>
void resize_file(const std::filesystem::path& p,
void resize_file(const std::filesystem::path& p, (since C++17)
std::error_code& ec) noexcept;
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.
p - path to resize
new_size - size that the file will now have
ec - out-parameter for error reporting in the non-throwing overload
The overload that does not take a std::error_code& parameter throws 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.
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.
demonstrates the effect of creating a sparse file on the free space
// Run this code
file_size returns the size of a file
space determines available free space on the file system