CODEWITHSUNDEEP
c++streamstdfstreamofstream
cweston
cweston

Reputation: 11647

std::ofstream, check if file exists before writing

I am implementing file saving functionality within a Qt application using C++.

I am looking for a way to check to see if the selected file already exists before writing to it, so that I can prompt a warning to the user.

I am using an std::ofstream and I am not looking for a Boost solution.

Upvotes: 38

Views: 89448

Answers (6)

return 0
return 0

Reputation: 4386

I suggest ifstream::good()

Here is an example of what using the fstream member function looks like:


  bool fileExists(const char *fileName)
  {
        ifstream inFile(fileName);
        return inFile.good();  
  }

The method is short and portable. When the code base is simple this is a good solution to use (no pun intended).

Upvotes: 42

Rico
Rico

Reputation: 6052

File Exists Function

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

  #include <sys/stat.h>

  /**
    * File Exist Function
    *
    * @desc Check if a file exists
    *
    * @param filename - the name of the file to check
    *
    * @return 'true' if it exist, otherwise returns false.
    * */
    bool fileExists(const std::string& filename){
      struct stat buf;

      if (stat(filename.c_str(), &buf) != -1){ return true; }

      return false;
    }

I find this solution to be much more tasteful than trying to open a file when you don't intend to stream in or out of it.


Upvotes: 72

Richard
Richard

Reputation: 8758

With std::filesystem::exists of C++17:

#include <filesystem> // C++17
#include <iostream>
namespace fs = std::filesystem;

int main()
{
    fs::path filePath("path/to/my/file.ext");
    std::error_code ec; // For using the noexcept overload.
    if (!fs::exists(filePath, ec) && !ec)
    {
        // Save to file, e.g. with std::ofstream file(filePath);
    }
    else
    {
        if (ec)
        {
            std::cerr << ec.message(); // Replace with your error handling.
        }
        else
        {
            std::cout << "File " << filePath << " does already exist.";
            // Handle overwrite case.
        }
    }
}

See also std::error_code.

In case you want to check if the path you are writing to is actually a regular file, use std::filesystem::is_regular_file.

Upvotes: 6

Bhagyesh Dudhediya
Bhagyesh Dudhediya

Reputation: 1864

One of the way would be to do stat() and check on errno.
A sample code would look look this:

#include <sys/stat.h>
using namespace std;
// some lines of code...

int fileExist(const string &filePath) {
    struct stat statBuff;
    if (stat(filePath.c_str(), &statBuff) < 0) {
        if (errno == ENOENT) return -ENOENT;
    }
    else
        // do stuff with file
}

This works irrespective of the stream. If you still prefer to check using ofstream just check using is_open().
Example: 

ofstream fp.open("<path-to-file>", ofstream::out);
if (!fp.is_open()) 
    return false;
else 
    // do stuff with file

Hope this helps. Thanks!

Upvotes: 2

NPE
NPE

Reputation: 500903

Try ::stat() (declared in <sys/stat.h>)

Upvotes: 5

HighCommander4
HighCommander4

Reputation: 52957

fstream file;
file.open("my_file.txt", ios_base::out | ios_base::in);  // will not create file
if (file.is_open())
{
    cout << "Warning, file already exists, proceed?";
    if (no)
    { 
        file.close();
        // throw something
    }
}
else
{
    file.clear();
    file.open("my_file.txt", ios_base::out);  // will create if necessary
}

// do stuff with file

Note that in case of an existing file, this will open it in random-access mode. If you prefer, you can close it and reopen it in append mode or truncate mode.

Upvotes: 10

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