Read from cin or a file

问题:

When I try to compile the code

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

gcc fails, complaining that operator= is private. Is there any way to set an istream to different values based on a condition?

回答1:

You can replace cin's streambuf with another, and in some programs this is simpler than the general strategy of passing around istreams without referring to cin directly.

int main(int argc, char* argv[]) {
  ifstream input;
  streambuf* orig_cin = 0;
  if (argc >= 2) {
    input.open(argv[1]);
    if (!input) return 1;
    orig_cin = cin.rdbuf(input.rdbuf());
    cin.tie(0); // tied to cout by default
  }

  try {
    // normal program using cin
  }
  catch (...) {
    if (orig_cin) cin.rdbuf(orig_cin);
    throw;
  }

  return 0;
}

Even though it's extremely rare to use cin after control leaves main, the above try-catch avoids undefined behavior if that's something your program might do.

回答2:

You could use a pointer for in, e.g.:

istream *in;
ifstream ifn;

if (argc==1) {
     in=&cin;
} else {
     ifn.open(argv[1]);
     in=&ifn;
}

回答3:

So, is it not complaining "no appropriate constructor available" ? Anyways, you can modify it as below.

void Read(istream& is)
{
    string line;
    while (getline(is, line))
        cout << line;
}

int main(int argc, char* argv[])
{
    if (argc == 1)
        Read(cin);
    else
    {
        ifstream in("sample.txt");
        Read(in);
    }
}

回答4:

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

#include <fstream>
#include <istream>
#include <iostream>

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'
  // ...
}