How to check if a file exists before creating a new file

Question

I want to input some contents to a file  but I d like to check first if a file with the name I wish to create exists  If so  I don t want to create any file  even if the file is empty     My attempt  bool CreateFile char name    char content          std  ofstream file name        if file            std  cout  lt  lt   This account already exists   lt  lt  std  endl          return false              file  lt  lt  content       file close         return true      Is there any way to do what I want

User · Answer

Try this  copied-ish from Erik Garrison  https   stackoverflow com a 3071528 575530    include  lt sys stat h gt   bool FileExists char  filename         struct stat fileInfo      return stat filename   amp fileInfo     0      stat returns 0 if the file exists and -1 if not

User · Answer

C  17  cross-platform  Using std  filesystem  exists and std  filesystem  is regular file    include  lt filesystem gt     C  17  include  lt fstream gt   include  lt iostream gt  namespace fs   std  filesystem   bool CreateFile const fs  path amp  filePath  const std  string amp  content        try               if  fs  exists filePath                         std  cout  lt  lt  filePath  lt  lt    already exists                return false                    if   fs  is regular file filePath                         std  cout  lt  lt  filePath  lt  lt    is not a regular file                return false                      catch  std  exception amp  e                std  cerr  lt  lt    func    lt  lt     An error occurred     lt  lt  e what            return false            std  ofstream file filePath       file  lt  lt  content      return true    int main         if  CreateFile  path to the file ext    Content of the file                     Your business logic

User · Answer

I just saw this test   bool getFileExists const TCHAR  file       return  GetFileAttributes file     0xFFFFFFFF

User · Answer

Assuming it is OK that the operation is not atomic  you can do   if  std  ifstream name          std  cout  lt  lt   File already exists   lt  lt  std  endl       return false    std  ofstream file name   if   file         std  cout  lt  lt   File could not be created   lt  lt  std  endl       return false           Note that this doesn t work if you run multiple threads trying to create the same file  and certainly will not prevent a second process from  interfering  with the file creation because you have TOCTUI problems   We first check if the file exists  and then create it - but someone else could have created it in between the check and the creation - if that s critical  you will need to do something else  which isn t portable     A further problem is if you have permissions such as the file is not readable  so we can t open it for read  but is writeable  it will overwrite the file    In MOST cases  neither of these things matter  because all you care about is telling someone that  you already have a file like that   or something like that  in a  best effort  approach

User · Answer

Looked around a bit  and the only thing I find is using the open system call  It is the only function I found that allows you to create a file in a way that will fail if it already exists  include  lt fcntl h gt   include  lt errno h gt   int fd open filename  O WRONLY   O CREAT   O EXCL  S IRUSR   S IWUSR   if  fd  lt  0         file exists or otherwise uncreatable      you might want to check errno    else        File is open to writing       Note that you have to give permissions since you are creating a file  This also removes any race conditions there might be

User · Answer

you can also use Boost    boost  filesystem  exists  filename      it works for files and folders    And you will have an implementation close to something ready for C  14 in which filesystem should be part of the STL  see here

User · Answer

As of C  17 there is   if  std  filesystem  exists pathname

User · Answer

The easiest way to do this is using ios    noreplace

User · Answer

Try  ifstream my file  test txt    if  my file        do stuff     From  How to check if a file exists and is readable in C     or you could use boost functions

[c++] How to check if a file exists before creating a new file

Examples related to c++

Examples related to file