Question

In C++ there is a standard library function called cout, which lets me send text to the console. I am sure you know that.

#include <iostream>
using std::cout;
cout << "Some Text " << 15 << " Other Text";

To do a linebreak at the end, I need to use endl.

cout << "Some Text " << 15 << " Other Text" << endl;

How can I write a function named coutl which behaves like cout but also adds a likebreak? I want to use the same syntax that cout uses, especially the << operator.

coutl << "Some Text " << 15 << " Other Text"; // coutl should add a linebreak

Answer 1

By creating a little proxy object which adds << endl in its destructor:

class AddEndl
{
public:
  AddEndl(std::ostream& os) : _holder(new Holder(os)) {}

  template <class T>
  friend std::ostream& operator<< (const AddEndl& l, const T& t)
  {
    return (l._holder->_os) << t;
  }

private:
  struct Holder {
    Holder (std::ostream& os) : _os(os) {}
    ~Holder () { _os << std::endl; }

    std::ostream& _os;
  };

  mutable std::shared_ptr<Holder> _holder;
}

Then you need a function so that you will get a temporary:

AddEndl wrap(std::ostream& os)
{
  return AddEndl(os);
}

This should then work:

wrap(std::cout) << "Hello";

UPDATE:

I move the destructor which adds a std::endl to an inner object owned by a std::shared_ptr<> so that the example doesn’t depend on Copy Elision anymore.