CODEWITHSUNDEEP
c++file
Paul Nathan
Paul Nathan

Reputation: 40299

ifstream: how to tell if specified file doesn't exist

I want to open a file for reading. However, in the context of this program, it's OK if the file doesn't exist, I just move on. I want to be able to identify when the error is "file not found" and when the error is otherwise. Otherwise means I need to quit and error.

I don't see an obvious way to do this with fstream.

I can do this with C's open() and perror(). I presumed that there was a fstream way to do this as well.

Upvotes: 74

Views: 141879

Answers (9)

hunght
hunght

Reputation: 1

if (!fs::exists(fileName))
{
     // error! file doesn't exist.
}

Upvotes: -1

Antonio
Antonio

Reputation: 20256

With C++17 you can use std::filesystem::exists.

Upvotes: -1

Cogwheel
Cogwheel

Reputation: 23217

Since the result of opening a file is OS-specific, I don't think standard C++ has any way to differentiate the various types of errors. The file either opens or it doesn't.

You can try opening the file for reading, and if it doesn't open (ifstream::is_open() returns false), you know it either doesn't exist or some other error happened. Then again, if you try to open it for writing afterwards and it fails, that might fall under the "something else" category.

Upvotes: 27

Arun
Arun

Reputation: 2373

Straight way without creating ifstream object.

if (!std::ifstream(filename))
{
     // error! file doesn't exist.
}

Upvotes: -7

user1633272
user1633272

Reputation: 2309

A better way:

std::ifstream stream;
stream.exceptions(std::ifstream::failbit | std::ifstream::badbit);
stream.open(fileName, std::ios::binary);

Upvotes: 4

LI Xuhong
LI Xuhong

Reputation: 2356

A simple way from http://www.cplusplus.com/forum/general/1796/

ifstream ifile(filename);
if (ifile) {
  // The file exists, and is open for input
}

Upvotes: 9

SwarthyMantooth
SwarthyMantooth

Reputation: 1857

EDIT: I've been notified that this does not necessarily indicate a file does not exist, as it may be flagged due to access permissions or other issues as well.

I know I'm extremely late in answering this, but I figured I'd leave a comment anyway for anyone browsing. You can use ifstream's fail indicator to tell if a file exists. 

ifstream myFile("filename.txt");
    if(myFile.fail()){
        //File does not exist code here
    }
//otherwise, file exists

Upvotes: 98

Khaled Alshaya
Khaled Alshaya

Reputation: 96849

I don't think you can know if "the file doesn't exist". You could use is_open() for generic checking:

ofstream file(....);
if(!file.is_open())
{
  // error! maybe the file doesn't exist.
}

If you are using boost you could use boost::filesystem:

#include <boost/filesystem.hpp>
int main()
{
    boost::filesystem::path myfile("test.dat");

    if( !boost::filesystem::exists(myfile) )
    {
        // what do you want to do if the file doesn't exist 
    }
}

Upvotes: 48

Erik Garrison
Erik Garrison

Reputation: 1717

You can use stat, which should be portable across platforms and is in the standard C library:

#include <sys/stat.h>

bool FileExists(string filename) {
    struct stat fileInfo;
    return stat(filename.c_str(), &fileInfo) == 0;
}

If stat returns 0, the file (or directory) exists, otherwise it doesn't. I assume that you'll have to have access permissions on all directories in the file's path. I haven't tested portability, but this page suggests it shouldn't be an issue.

Upvotes: 6

Related Questions