Std::Ofstream, Check If File Exists Before Writing

std::ofstream, check if file exists before writing

This is one of my favorite tuck-away functions I keep on hand for multiple uses.

#include <sys/stat.h>
// Function: fileExists
/**
 *  Check if a file exists
 *
 * @param[in] filename - the name of the file to check
 *
 * @return    true if the file exists, else false
*/
bool fileExists(const std::string& filename)
{
    struct stat buf;
    if (stat(filename.c_str(), &buf) != -1)
    {
        return true;
    }
    return false;
}

I find this much more tasteful than trying to open a file if you have no immediate intentions of using it for I/O.

How to check if a file exists in C++ with fstream::open()

It says it sets the failbit if the file couldn't be opened. So you can check for that bit:

fileStream.open("logs.txt");
if (fileStream.fail()) {
    // file could not be opened
}

Actually, just if (fileStream) would work here as well, since ios (a base class of ifstream, ofstream, and fstream) has a conversion operator to bool.

Don't worry about the failure exception. You can request exceptions to be thrown on failure by calling ios::exceptions but by default exceptions are not thrown on failure.

Note that this doesn't tell you why the file couldn't be opened. It could be that the file didn't exist, that a directory in the path didn't exist, you don't have permission to open the file, your program has reached the limit on the number of files it can open, and so on. There is no portable way to determine the reason.

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

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.

Fastest way to check if a file exists using standard C++/C++11,14,17/C?

Well I threw together a test program that ran each of these methods 100,000 times, half on files that existed and half on files that didn't.

#include <sys/stat.h>
#include <unistd.h>
#include <string>
#include <fstream>

inline bool exists_test0 (const std::string& name) {
    ifstream f(name.c_str());
    return f.good();
}

inline bool exists_test1 (const std::string& name) {
    if (FILE *file = fopen(name.c_str(), "r")) {
        fclose(file);
        return true;
    } else {
        return false;
    }   
}

inline bool exists_test2 (const std::string& name) {
    return ( access( name.c_str(), F_OK ) != -1 );
}

inline bool exists_test3 (const std::string& name) {
  struct stat buffer;   
  return (stat (name.c_str(), &buffer) == 0); 
}

Results for total time to run the 100,000 calls averaged over 5 runs,

fstream file;
file.open("data.txt");

if (fileStream.fail()) 
{
    ofstream file("data.txt");
}
file.open("data.txt");