C++ redirecting input stream

I am working on a project for my advanced programming class.  It is an expression interpreter.  I have a good portion of what I need.  I feel like I have gotten a lot done in a short amount of time and am now having an issue with redirecting the input stream to be a file, as opposed to the typical keyboard input.  In the present state of my program the user is asked for input:

int main()
{
	Expression* expression;
	char paren, comma;
	cout << "Enter expression: ";
	cin >> paren;
	expression = SubExpression::parse();
	cin >> comma;
	parseAssignments();
	cout << "Value = " << expression->evaluate() << endl;
	cin >> comma; // to wait for user input; allows the user to see what was printed before the window closes
	return 0;
}

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I understand that I can use this method to read the input and display the lines of the input:

int main() {
	string line;
	ifstream myfile;
	myfile.open("CMSCProject3_Test.txt");
	if (myfile.is_open())
	{
		while (getline(myfile, line))
		{
			cout << line << '\n';
		}
		myfile.close();
	}
	else cout << "Unable to open file";

	return 0;
}

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I am unsure of how to combine these two to interact with the program that I have created.  Where do I move the

 
        cin >> paren;
	expression = SubExpression::parse();
	cin >> comma;
	parseAssignments(); 

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to so that it will operate?  Any assistance at all is greatly appreciated.  I feel like the answer is right in front of me, however I have not dealt with file input in C++ prior to this.  Thank you in advance for your assistance.
AutkastAsked:
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jkrConnect With a Mentor Commented:
Yes, there is a way. Actually two ways, the more complex without global variables where you pass the file to each function involved as a parameter. The other one is a bit simpler, create a global variable as a surrogate for 'cin'. 'cin is usually declared as 'extern istream cin;', i.e. also a global variable. Now you'd create your own like

istream myin;

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and asign it accordingly:

  if (!interactive) // don't use 'cin'
  {
	ifstream myfile;
	myfile.open("CMSCProject3_Test.txt");
  	if (!myfile.is_open())
	 {
            return -1; // or any appropriate error code to indicate 'Could not open...'
        }
 
        myin = myfile;
        myin >> paren;
	expression = SubExpression::parse();
	myin >> comma;
	parseAssignments(); 
           
// etc.                       
  }
  else
  {
     myin = cin; // use cin instead of a file
  }                                            

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jkrConnect With a Mentor Commented:
You'd basically replace 'cin' in your original code with the ifstream you opened, e.g. like

	ifstream myfile;
	myfile.open("CMSCProject3_Test.txt");
	if (!myfile.is_open())
	{
            return -1; // or any appropriate error code to indicate 'Could not open...'
        }
 
        myfile >> paren;
	expression = SubExpression::parse();
	myfile >> comma;
	parseAssignments(); 
           
// etc.                       

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AutkastAuthor Commented:
Thank you for the prompt response!  I get a hang up when I am calling those 4 lines of code now though.  I'm assuming my problem is now in the other files associated with this main function (such as subexpression, parse, etc.).  Will I need to alter the input for these files as well to be from the text file object?  For example my parse:

string parseName()
{
    char alnum;
    string name = "";

    cin >> ws;
    while (isalnum(cin.peek()))
    {
        cin >> alnum;
        name += alnum;
    }
    return name;
}

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Is there a certain way I make this new input viewable to the entire program that is accessed by this main function?
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AutkastAuthor Commented:
Thank you for the help this evening, greatly appreciated.
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jkrCommented:
You're most welcome ;o)
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jkrCommented:
Just as a side note, the method of either passing thei fstream object to related methods or to the constructor of 'SubExpression' would be the preferred way...
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