C++ Ifstream

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Here are some common tasks with their implementations. There is a good reference for this, but it doesn't include these examples.

• Read in values safely - If you want to read in a double you would like to be able to check for errors ("asdf" != good_value). You can use the ios::fail() function to do this.

  double v1, v2, v3;
  in >> v1 >> v2 >> v3;
  if (!in.fail()) {
    // The values were properly parsed
    do_stuff(v1, v2, v3);
  } else {
    // There was a problem.  Depending on what you want to do, you can simply
    ignore it; however, you need to at least clear the error state.
    in.clear();
  }
  

• Don't read in one too many things - The following code is quite tempting, but will produce the wrong results.

  #include 
  #include 
  #include 
  
  using namespace std;
  
  int main(int argc, char* argv[]) {
  
    if (argc < 2) {
      cerr << "Usage: "<< argv[0] << " filename\n";
      return 1;
    }
  
    char* filename = argv[1];
    ifstream in(filename);
    int numwords = 0;
    while(in.good()) {
      string token;
      in >> token;
      numwords++;
    }
  
    cout << filename << " had " << numwords << " words in it\n";
    
    return 0;
  }
  

  This will set numwords to be one extra than it should be. The problem is that the stream won't hit the end of file (EOF) until you try to read something that isn't there. There are a couple of different ways around this. The first involves adding an extra check before you increment numworkds.

    while(in.good()) {
      string token;
      in >> token;
      if (!in.eof()) numwords++;
    }
  

  The second forces the reading of the next token before the loop iterates.

    string token;
    in >> token;
    while(in.good()) {
      numwords++;
      in >> token;
    }
  

  Depending on what you are trying to do, the first method has better scoping. You can also replace in.eof() with in.good() to catch other errors, but it seems kind of odd that you would have to have two calls to check the state of the stream. By rewriting the loop you can get down to a single check. This check could also be expanded to distinguish between bad reads or the end of the file.

    for(;;) {
      string token;
      in >> token;
      if (in.good()) {
        numwords++;
      } else if (in.eof()) {
        break;
      } else {
        cerr << "Error reading string\n";
        break;
      }
    }