// How to overload << to print a self-made class object
#include <iostream>
#include <vector>
#include <iterator>
#include <iomanip>
#include <fstream>

class MyClass
{
  double a,b;
  std::vector<double> v;
public:
  // universal initialization
  MyClass(double a_,double b_,std::vector<double> v_): a{a_},b{b_},v{v_}{}
  // overload << for MyClass objects;  make << a friend to grant access toprivate data
  friend std::ostream& operator <<(std::ostream& , const MyClass&);
};

std::ostream& operator<<(std::ostream & os, const MyClass& obj)
{
  using namespace std;
  os<<fixed<<setprecision(8); // some I/O manipulation 
  os<<"a="<<obj.a<<" b="<<obj.b<<endl;
  os<<"v=";
  for(auto ele:obj.v) os<<ele<<" ";
  return os;
}

int main()
{
  using namespace std;
  MyClass obj{2.2,3.1,{1.0,2.0,3.0}}; // universal initialization
  cout<<obj<<endl;  // screen output uses overloaded <<
  ofstream out("MyClass.out");
  out<<obj<<endl;  // file output uses overloaded <<
}