Using istream_iterator and reading from standard input or file

Question

I'm writing in Microsoft Visual C++ and I'd like my program to either read from standard input or a file using the istream_iterator. Googling the internets hasn't shown how simple I think it must be. So for example, I can write this pretty easily and read from standard input:

#include <iostream>
#include <string>
#include <iterator>

using namespace std;

int main()
{
   istream_iterator<string> my_it(cin);
   for (; my_it != istream_iterator<string>(); my_it++)
      printf("%s\n", (*my_it).c_str());
}

Or I can write this and read from a file:

#include <iostream>
#include <string>
#include <iterator>
#include <fstream>

using namespace std;

int main(int argc, char** argv)
{
   ifstream file(argv[1]);
   istream_iterator<string> my_it(file);
   for (; my_it != istream_iterator<string>(); my_it++)
      printf("%s\n", (*my_it).c_str());
}

But how do I combine these two so that a simple (argc == 2) check lets me initialize my input stream iterator with either a file stream or stdin and go on about my merry way?

Answer 1

You can assign to the iterator after constructing it:

int main(int argc, char** argv)
{
   ifstream file;
   istream_iterator<string> my_it;

   if(argc == 2) {
      file.open(argv[1]);
      my_it = istream_iterator<string>(file);
   } else {
      my_it = istream_iterator<string>(cin);
   }
}

Answer 2

At first glance, the simplest solution would be to use the ternary operator ?: like this:

istream_iterator<string> my_it( (argc == 2) ? ifstream(argv[1]) : cin );

However, that won't quite work because it constructs a temporary ifstream object, which will be destroyed at the end of the statement. So you need a way of conditionally creating an ifstream, and conditionally destroying it after the for loop. std::auto_ptr<> fits the bill. Thus:

auto_ptr<ifstream> file((argc == 2) ? new ifstream(argv[1]) : NULL);
istream_iterator<string> my_it( (argc == 2) : *file : cin);
for (; my_it != istream_iterator<string>(); my_it++)
   printf("%s\n", (*my_it).c_str());

A different, probably cleaner solution would be to move the iteration to a separate function that takes istream&.

I've seen this problem before online, covered by one of the C++ greats. Unfortunately, I don't remember exactly where, or by whom! I think it was on DDJ, maybe Sutter or Alexandrescu?

Answer 3

This small snippet will give you an istream input that can be either a file or std::cin.

std::ifstream filestream;
if ( use_file )
    filestream.open( ... );
std::istream &input = use_file ? filestream : std::cin;

You may now use input without worrying which source the input is coming from.