std::filesystem::is_regular_file() vs::exists() - 什么算作常规文件

Question

I am using Visual Studio 2017 C++ WITH MFC

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I have a program in MFC that collects filepaths as string s and adds them to a zip file.

I want to add an error check, where I check if a file exists before trying to add it. If it exists, perfect, I add the file to the zip . If not, not a problem, I continue to the next filepath.

I came across std::filesystem ( here ) and I see two different functions that I think can work: is_regular_file() ( here ) and exists() ( here ).

However, I am not sure which of the two to use. The file types I will be zipping vary from .txt to .zip , and a lot of arbitrary file types.

From my research, they appear to be similar, both returning a bool value.

What is the difference between the two functions and which is the better one to use?

Furthermore, from my understanding, the library is relatively new and might not be suitable for use in MFC . Is this true? And if so, what else could I use to check if given a file path, that file exists on the computer?

Thank you in advance: :D

Answer 1

tl;dr: use std::filesystem::is_regular_file() , std::filesystem::is_symlink() and std::filesystem::is_directory() .

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There are several file types defined in std::filesystem :

Constant    Meaning
none        indicates that the file status has not been evaluated yet, or an error occurred when evaluating it
not_found   indicates that the file was not found (this is not considered an error)
regular     a regular file
directory   a directory
symlink     a symbolic link
block       a block special file
character   a character special file
fifo        a FIFO (also known as pipe) file
socket      a socket file
implementation-defined  an additional implementation-defined constant for each additional file type supported by the implementation (e.g. MSVC STL defines junction for NTFS junctions)
unknown     the file exists but its type could not be determined 

So in first glance, std::filesystem::exists() would be what you want.

But you're adding files to a ZIP archive, and AFAICT, ZIP only supports regular files, symlinks, and directories, so that's what you should use.

You wouldn't archive sockets or NTFS junctions even if they existed and were given in command line.

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To clarify:

A "regular file" is a file that has data that is (a) stored by the filesystem and (b) has no semantics assigned to it by the filesystem - ie the filesystem doesn't know how to read the file or act on it.

For example, a MS Word document is something that MS Word understand, but the filesystem only knows that it's a bunch of bytes.
A symlink, on the other hand, is something the filesystem stores, and knows how to act on (follow the link).
A block special file is something that is not actually stored by the filesystem, but rather just a piece of metadata that other parts of the OS act on.
A ZIP archive, or RAR, are files that the filesystem stores. But it doesn't know how to read them or act on them, so they are regular files.
In the old Windows Explorer, the "shortcuts" on your desktop were small files stored by the filesystem, but it was Explorer that knew how to read them - they weren't symbolic links, they were regular files. (Maybe that's still the case, I haven't checked recently.)