Question

Newbie C++ programmer here. I’m trying to write a command line application that takes two arguments, an input file and an output file. However, if the input file or the output file name is “-“, I need the program to read/output to standard input/output instead. My problem is that in C++, I don’t know how to do this without the compiler not knowing that the input/output streams are initialized. Here’s the code I have.

if(argv[1] == "-") {
  istream input;
  cout << "Using standard input" << endl;
}
else {
  ifstream input;
  cout << "Using input file " << argv[1] << endl;
  input.open(argv[1], ios::in);
  if(!input) {
    cerr << "Cannot open input file '" << argv[1]
    << "', it either does not exist or is not readable." << endl;
    return 0;
  }
}

if(argv[2] == "-") {
  ostream output;
  cout << "Using standard output" << endl;
}
else {
  ofstream output;
  cout << "Using output file " << argv[2] << endl;
  output.open(argv[2], ios::out);

  if(!output) {
    cerr << "Cannot open output file '" << argv[2] << "' for writing."
    << " Is it read only?" << endl;
    return 0;
  }
}

From here I cannot call the operator >> on input, because, I’m guessing, the compiler doesn’t know it’s been initialized.

Answer 1

You can use a reference to a stream, and then initialize it to refer to either a file stream or standard input or output. The initialization has to happen in a single command, though, so you’ll have to declare the file streams even if you don’t use them.

ifstream file_input;
istream& input = (strcmp(argv[1], "-") == 0) ? cin : file_input;

ofstream file_output;
ostream& output = (strcmp(argv[2], "-") == 0) ? cout : file_output;

Notice the & in the declarations of input and output. They indicate that we’re not declaring a separate stream object, but rather just declaring a reference to some other stream object, which we select conditionally based on the value of argv[x].

Then you can open the files, if you need to. The drawback is that we need to check for “-” strings twice instead of just once for each input or output.

if (strcmp(argv[1], "-") == 0) {
  cout << "Using standard input" << endl;
} else {
  cout << "Using input file " << argv[1] << endl;
  file_input.open(argv[1]);
  if (!file_input) {
    cerr << "Cannot open input file '" << argv[1]
         << "', it either does not exist or is not readable." << endl;
    return 1;
  }
}

Thereafter, you can read from input and write to output, and the files or the standard I/O streams will be used.

---

Note the other changes I made to your code. First, I call strcmp instead of using the == operator; the operator doesn’t do what you think it does when comparing a char* with a literal. Next, when opening the file fails, I return 1 instead of 0. Zero indicates a successful program, whereas non-zero tells the OS that the program failed.