std::experimental::filesystem::resize_file - cppreference.com (original) (raw)

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.

[edit] 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

[edit] Return value

(none)

[edit] Exceptions

The overload that does not take an error_code& parameter throws filesystem_error on underlying OS API errors, constructed with p as the first argument and the OS error code as the error code argument. std::bad_alloc may be thrown if memory allocation fails. The overload taking an 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. This overload has

[edit] 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.

[edit] Example

Demonstrates the effect of creating a sparse file on the free space.

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

Possible output: