Question

Is there a common interface for cin and file input?

I want to make a program that has an optional parameter

prog [input-file]

If an input file is specified, then it should read from the file, and if not, it should read from cin.

From what I can tell, they both implement istream. How would you set up it so that I could do something like in >> var, where in is an istream.

Answer 1

#include <iostream>
#include <fstream>

int main(int argc, char **argv)
{
    std::ifstream f;
    if (argc >= 2) {
        f.open(argv[1]);
    }
    std::istream &in = (argc >= 2) ? f : std::cin;

    // use in here
}

You could shift some of this work into a helper class to make it clearer what’s going on (note that this has slightly different behavior in the case where the file can’t be opened):

#include <iostream>
#include <fstream>

class ifstream_or_cin_t {
    std::ifstream f;

public:
    ifstream_or_cin_t(const char *filename)
    {
        if (filename) {
            f.open(filename);
        }
    }

    operator std::istream &() { return f.is_open() ? f : std::cin; }
};

static void do_input(std::istream &in)
{
    // use in...
}

int main(int argc, char **argv)
{
    do_input(
        ifstream_or_cin_t((argc >= 2) ? argv[1] : NULL));
}