Read from cin or a file

Question

When I try to compile the code

istream in;
if (argc==1)
        in=cin;
else
{
        ifstream ifn(argv[1]);
        in=ifn;
}

gcc fails,

Answer 1

You cannot affect streams like this. What you want to achieve can be obtained using a pointer to an istream though.

#include 
#include 
#include 

using namespace std;

int main(int argc, char *argv[])
{
  istream *in;
  // Must be declared here for scope reasons
  ifstream ifn;

  // No argument, use cin
  if (argc == 1) in = &cin;
  // Argument given, open the file and use it
  else {
    ifn.open(argv[1]);
    in = &ifn;
  }
  return 0;

  // You can now use 'in'
  // ...
}