Writing void into a file with c++ - c++

Question

i have the following code
void print(int & a, double & b, string & c)
{
cout << setprecision(2) << fixed;
const double GPA = a/b;
if(c == "Y")
{
cout << "\n\nTotal number of credit hours: " << a << endl;
}
else
{
cout << "\n*** Grades are being held for not paying the tuition. ***"
}
}
How can I write the cout in print(int, double, string) into a text file without tampering with print(int, double, string);? I tried something like this
ofstream file;
file.open("file.txt");
file << print(a,b,c);
file.close();
cout << "file created" << endl;
but this doesn't compile. Why not, and how do I fix it?

Answer 1

The way you've written it, your print() function is not capable of outputting to any given stream. This is because it hard-codes the stream that it writes to as cout.
If you want it to be able to write to any given stream, you have to parameterize the stream as another function parameter. For (1) convenience and (2) compatibility with existing code that assumes print() only takes three arguments and writes to cout, you can make the new parameter optional by defaulting it to cout:
void print(int& a, double& b, string& c, ofstream& os=cout) {
os << setprecision(2) << fixed;
const double GPA = a/b;
if (c == "Y") {
os << "\n\nTotal number of credit hours: " << a << endl;
} else {
os << "\n*** Grades are being held for not paying the tuition. ***";
}
}
Then you can call it as follows:
print(a,b,c,file);
The reason why your code doesn't compile is that you cannot pass void as a function argument or operator operand. When a function is declared as returning void, that means it does not return anything at all. There is no data returned by print() to stream to the ofstream. The streaming takes place inside the function, so it is only there that you can select the stream to which the output will be written.

Answer 2

bgoldst's answer solves the problem as asked, but I recommend a completely different solution. Stick your data in a class that has operator<< overloaded.
struct class_results {
int credits;
double GP_total;
bool tuition_paid;
};
std::ostream& operator<<(std::ostream& out, const class_results& c) {
if (c.tuition_paid) {
const double GPA = c.credits/c.GP_total;
out << "Total number of credit hours: ";
out << setprecision(2) << fixed << c.credits<< '\n';
} else
out << "\n*** Grades are being held for not paying the tuition. ***"
return out;
}
Then usage is slightly more normal:
class_results results = {num_credits,GPTottal,tuition};
ofstream file;
file.open("file.txt");
file << results;
file.close();
cout << "file created" << endl;

Answer 3

How can I write the cout in print(int, double, string) into a text file without tampering with print(int, double, string);?
You can't.
The function print is broken and you can't do what you want without fixing it.