Reading from cin or file

istream.cc

#include <iostream>     // std::cout
#include <fstream>      // std::ifstream
#include <string>

void reading()
{
	std::string filename = ... ;
	
	std::ifstream ifs;                                                                                                        
        if(filename.size()>0) ifs.open(filename);                                                                                 
        std::istream &is = ifs.is_open() ? ifs : std::cin;                                                                        
        // read inputs                                                                                                            
        string line;                                                                                                        
        while(getline(is, line)) {                                                                                                
                ...                                                                                          
        }                                                                                                                         
        if(ifs.is_open()) ifs.close();      
	
}