How can I provide printing for an entire hierarchy of classes?, C++ FAQ

C++ FAQ / Section 15 / FAQ 15.11

Section 15:

15.1	Advantages of `<iostream>` over `<cstdio>`?
15.2	Understanding infinite loop when the input is invalid?
15.3	How can I get `std::cin` to skip invalid input characters?
15.4	How does that funky `while (std::cin >> foo)` syntax work?
15.5	Why does my input seem to process past the end of file?
15.6	Why is my program ignoring my input request after the first iteration?
15.7	Should I end my output lines with `std::endl` or `'\n'`?
15.8	How can I provide printing for my `class` `Fred`?
15.9	Using a `printOn()` method vs. a `friend` function?
15.10	How can I provide input for my `class` `Fred`?
15.11	Providing printing for an entire hierarchy of classes?
15.12	How can I open a stream in binary mode?
15.13	"Reopening" `std::cin` and `std::cout` in binary mode?
15.14	How can I write/read objects of my class to/from a data file?
15.15	Sending objects over a network?
15.16	Backslashes in filenames?
15.17	How to capture key presses before the ENTER key?
15.18	How to turn off screen echo on keyboard input?
15.19	How can I move the cursor around on the screen?
15.20	How can I clear the screen?
15.21	How can I change the colors on the screen?
15.22	How can I print the numeric value of a `char` or `char*`? Updated!

[15.11] How can I provide printing for an entire hierarchy of classes?

Provide a friend operator<< that calls a protected virtual function:

class Base {
public:
  friend std::ostream& operator<< (std::ostream& o, Base const& b);
  ...
protected:
  virtual void printOn(std::ostream& o) const = 0;  ← or plain virtual; see below
};

inline std::ostream& operator<< (std::ostream& o, Base const& b)
{
  b.printOn(o);
  return o;
}

class Derived : public Base {
public:
  ...
protected:
  virtual void printOn(std::ostream& o) const;
};

void Derived::printOn(std::ostream& o) const
{
  ...
}

The end result is that operator<< acts as if it were dynamically bound, even though it's a friend function. This is called the Virtual Friend Function Idiom.

Note that derived classes override printOn(std::ostream&) const. In particular, they do not provide their own operator<<.

As to whether Base::printOn() is plain virtual or pure virtual, consider making it a plain virtual (without the "= 0") if you can implement that function with code that would otherwise be repeated in two or more derived classes. However if Base is a ABC with little or no member data, you might not be able to provide a meaningful definition for Base::printOn() and you should make it pure virtual. If you're not sure, make it pure virtual, at least until you get a better handle on the derived classes.