函数：引用file类对象及io类对象作为参数打印文本及显示文本

#include <iostream>
#include <fstream>
#include <cstdlib>

using namespace std;

void file_it(ostream & os, double fo, const double fe[], int n);
const int LIMIT = 5;

int main(void)
{
	ofstream fout;
	const char * fn = "ep-data.txt";
	fout.open(fn);
	if(!fout.is_open())
	{
		cout << "Can't open " << fn << ". Bye.\n";
		exit(EXIT_FAILURE);
	}
	
	double objective;
	cout << "Enter the focal length of your "
	        "telescope objective in mm: ";
	cin >> objective;
	double eps[LIMIT];
	cout << "Enter the focal length, in mm, of " << LIMIT
	     << " eyepieces:\n";
	for(int i=0; i<LIMIT; i++)
	{
		cout << "Eyepiece #" << i+1 << ": ";
		cin >> eps[i];
	}
	file_it(fout, objective, eps, LIMIT);
	file_it(cout, objective, eps, LIMIT);
	cout << "Done\n";
	return 0;
}
void file_it(ostream & os, double fo, const double fe[], int n)
{
	ios_base::fmtflags initial;
	initial = os.setf(ios_base::fixed);
	os.precision(0);
	
	os << "Focal length of objective: " << fo << " mm\n";
	os.setf(ios::showpoint);
	os.precision(1);
	os.width(12);
	os << "f.1. eyepiece";
	os.width(15);
	os << "magnification" << endl;
	
	for(int i=0; i<n; i++)
	{
		os.width(12);
		os << fe[i];
		os.width(15);
		os << int (fo/fe[i] + 0.5) << endl;
	}
	os.setf(initial);
}