How do I output the cout output from a method into an ofstream file in c++?

Question

I have a main function that prints a sudoku game to the console as I play. I call a print board method which outputs the board along with a 2D array with the numbers. I've been modifying the game to output to the file after every cout:

cout << "puzzle created" << endl;
output << "puzzle created" << endl;

Then the problem is that there is an error when I try the same with a method:

sudoku.printBoard(); //method to print board to console

I can't simply say:

output << sudoku.printBoard();

and I can't say:

output << "number";     

in the method in the board.cpp file either.

Does anyone know any way around this?

Answer 1

This is the problem with print() member functions, and the reason you shouldn't write them. At least, not like this. You've locked yourself into printing to stdout , and now that you want to print somewhere else instead, you're stuck.

Instead, have a function that prints where you tell it to . This may be stdout or a file or whatever.

The function will be declared like this:

void printBoard(std::ostream& os);

and inside its definition, you will use os rather than std::cout .

Then your code will look like this:

sudoku.printBoard(std::cout);
sudoku.printBoard(output);

It works because both std::cout and output are of types that derive from std::ostream .

---

If you need the cout variant a lot, and you don't want to provide the argument every time because it's getting messy, simply provide an overload:

void printBoard()
{
    printBoard(std::cout);
}

Now you can still write:

sudoku.printBoard();

Whether this is more or less confusing for developers on your project is for you to decide.

---

If you don't have other things to print, so that this is the only function of its kind within the type of sudoku , a more idiomatic approach would be to create an operator<< for that type:

std::ostream& operator<<(std::ostream& os, const SudokuBoard& board)
{
    board.printBoard(os);
    return os;
}

Now you can use it like this:

std::cout << sudoku;
output << sudoku;

Answer 2

Pass ofstream as a reference to your funtion or simply return string from your function and print it in main() or callee function.

In 2nd case you go like this

string f()
{
   ostringstream os;
   os<<"whatever I want to print in output";
   ....
   ...
   return os.str();
}
int main() or //womain() from wherever you call
{
   output<<f()<<endl;
}

Answer 3

Make a class that takes two ostreams in the constructor. One of these can be std::cout. Store these in the class as references.

Create output stream operators (<<) and use those to write to the two streams.

To use this in other classes pass it as a reference in their constructor (look up dependency injection).