C++ cin with file instead of user input

Question

I have looked up what feels like every resource out there and I can't seem to find a solid answer to this question. Perhaps it's obvious, I am still new to C++.

I had the following functional main method:

int main()
{
    char firstChar, secondChar;
    cin >> firstChar;
    cin >> secondChar;
    cout << firstChar << " " << secondChar;

    system("pause"); // to wait for user input; allows the user to see what was printed before the window closes
    return 0;
}

This will cause the console to wait for input. The user inputs something. In this case (test). The output is:

( t

I would like to change this so that the input comes from a file and can execute the same way for each line rather than just once.

I tried many variations of the following:

int main(int argc, char* argv[])
{
    ifstream filename(argv[0]);
    string line;
    char firstChar, secondChar;
    while (getline(filename, line))
    {
        cin >> firstChar;  // instead of getting a user input I want firstChar from the first line of the file.
        cin >> secondChar; // Same concept here.
        cout << firstChar << " " << secondChar;
    }

    system("pause"); // to wait for user input; allows the user to see what was printed before the window closes
    return 0;
}

This merely runs the while loop once for every line in the file, but still requires input into the console and in no way manipulates the data in the file.

File contents:

(test)
(fail)

Desired automatic output (without making the user enter (test) and (fail) manually:

( t
( f

Solution

Final Edit

After seeing the input I would do something like this

int main(int argc, char* argv[])
{
    ifstream exprFile(argv[1]); // argv[0] is the exe, not the file ;)
    string singleExpr;
    while (getline(exprFile, singleExpr)) // Gets a full line from the file
    {
        // do something with this string now
        if(singleExpr == "( test )")
        {

        }
        else if(singleExpr == "( fail )") etc....
    }

    return 0;
}

You know what the full input is from the file so you can test the whole string at a time instead of character by character. Then just act accordingly once you have this string

Comments

[leigero]

There is one expression per line, but they can be rather complex so it's not so simple as just x + y. there can be nested ones as well etc. Previously I just handled this by extracting characters with cin >> charVariable and that would pull out the leftmost character. When it is a string, and not input, it changes everything.

[const_ref]

Please edit your question to show an example of the file layout

[leigero]

using cin >> charVariable causes the console to sit there waiting on user input. I want to use this same concept with an existing string. Something like singleExpr >> charVariable but that doesn't work.

[const_ref]

No but you can iterate over the string character by character. Please paste in a copy of your file and I will edit my answer accordingly

[const_ref]

@leigero See my edit for iterating over a string character by character. It sounds like behaviour like this that you desire

[const_ref]

cppreference.com is the preferred site for referencing. The site you mention is rather poor

[leigero]

With the edited question's file contents paren = filename.get(); prints out a dollar sign. The file's contents are (test)

[Abhishek Bagchi]

Works for me. I get (, and not $