Checking if a file opened successfully with ifstream

Question

I have the following that will open a file for reading. However, I want to check to make sure that the file was open successfully, so I am using the fail to see if the flags have been set. However, I keep getting the following error:

I am new to C++, as I am coming from C. So not sure I understand this error:

cannot call member function ‘bool std::basic_ios<_CharT, _Traits>::fail() const [with _CharT = char, _Traits = std::char_traits]’ without object

Code:

int devices::open_file(std::string _file_name)
{
    ifstream input_stream;

    input_stream.open(_file_name.c_str(), ios::in);

    if(ios::fail() == true) {
        return -1;
    }

    file_name = _file_name;

    return 0;
}

I prefer using `fin` (*file input*, similar to how `cin` is *console input*) instead of `input_stream`, since it's easier to write and it is perfectly understandable what it does. — H-005, Sep 04 '20 at 17:03

Sven · Accepted Answer · 2011-06-06T17:19:49.127

27

You can simply do this:

int devices::open_file(std::string _file_name)
{
    ifstream input_stream;    
    input_stream.open(_file_name.c_str(), ios::in);
    if(!input_stream)
    {
        return -1;
    } 
    file_name = _file_name;
    return 0;
}

fail() is not a static method, you must call it on an instance not a type, so if you want to use fail(), replace !input_stream with input_stream.fail() in my code above.

I do have to wonder what you're trying to achieve here. You're opening the file and immediately close it again. Are you simply trying to check if the file exists?

edited Jun 06 '11 at 17:19

answered Jun 06 '11 at 16:51

Sven

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Hello, thanks for the answer. HOwever, I am just wondering why the fail flag never worked for me? – ant2009 Jun 06 '11 at 16:53
1

Using `input_stream.fail()` should have the same effect as `!input_stream`. As I said, you can't use `ios::fail()` because it's an instance method, not a static method. – Sven Jun 06 '11 at 16:57
3

Please, why use `ios::in` flag with `ifstream`? – nn0p Mar 30 '15 at 16:39

score 7 · Answer 2 · answered Feb 05 '17 at 21:27

you can also use std::ifstream::is_open. Returns true if a file is open and associated with this stream object.

// ifstream::is_open
#include <iostream>     // std::cout
#include <fstream>      // std::ifstream

int main () {
  std::ifstream ifs ("test.txt");

  if (ifs.is_open()) {
    // print file:
    char c = ifs.get();
    while (ifs.good()) {
      std::cout << c;
      c = ifs.get();
    }
  }
  else {
    // show message:
    std::cout << "Error opening file";
  }

  return 0;
}

http://www.cplusplus.com/reference/fstream/ifstream/is_open/

score 5 · Answer 3 · answered Jun 06 '11 at 16:53

5

Your error is because you are using ios::fail() as a static method when it is actually a member method.

if (input_stream.fail())
{
    ...
}

answered Jun 06 '11 at 16:53

user7116

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score 3 · Answer 4 · answered Jun 06 '11 at 16:53

3

You have to call fail() on the stream object. A more idiomatic way of doing this is:

input_stream.open(_file_name.c_str(), ios::in);

if( ! input_stream ) {
    return -1;
}

answered Jun 06 '11 at 16:53

Checking if a file opened successfully with ifstream

4 Answers4

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