Can I redirect an ifstream to cin?

Question

I have a program that reads input from the terminal and the stream from cin is used in multiple classes for parsing at various levels.

Instead of using cin for getting the data from the terminal, I want to read in a file that has the information I need to parse, but I don't want to modify all my header and .cpp files to accept an ifstream& parameter.

To keep the existing code in place I'm trying to simply redirect the ifstream to cin, but don't know how.

So assume I have the following in main:

ifstream inFile("myfile.txt", ifstream::io);

string line;
while(getline(inFile, line)) 
{
  char firstChar;
  inFile >> firstChar;
  cout << firstChar;
  inFile.ios::rdbuf(cin.rdbuf());
  Continue myFile;
} 

In my continue.cpp I'm just doing:

Continue()
{
   string line;
   cin >> line;
   cout << "---remaining line: " << line << "\n";
}

However it's only printing the first char from main.

Answer 1

Simply swap pointers to std::streambuf:

ifstream file("myfile.txt");
string line;
if (file.is_open()) {
    cin.rdbuf(file.rdbuf());  // swap
    cin >> line;
}
std::cout << line;

Answer 2

istream objects read from a std::streambuf, and that can be swapped in an out. The relevant member function is .rdbuf