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 << "This account already exists" << std::endl;
        return false;
     }
     file << content;
     file.close();
     return true;
}

Is there any way to do what I want?

Answer 1

Assuming it is OK that the operation is not atomic, you can do:

if (std::ifstream(name))
{
     std::cout << "File already exists" << std::endl;
     return false;
}
std::ofstream file(name);
if (!file)
{
     std::cout << "File could not be created" << 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.

Answer 2

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).

Answer 3

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.

Answer 4

Try this (copied-ish from Erik Garrison: https://stackoverflow.com/a/3071528/575530)

#include <sys/stat.h>

bool FileExists(char* filename) 
{
    struct stat fileInfo;
    return stat(filename, &fileInfo) == 0;
}

stat returns 0 if the file exists and -1 if not.