Why is ranges::ostream_iterator default-constructible?

Question

_{This question follows a discussion in the comments here.}

In Eric Niebler\'s ranges-v3 library (which is sort-of becoming part of the standard for C++20),

Answer 1

This follows the Elements of Programming design philosophy of how types should behave. If you've heard the phrase "do as the ints do", that is that philosophy -- types should be Regular. And the EoP definition of Regular is:

T’s computational basis includes equality, assignment, destructor, default constructor, copy constructor, total ordering (or default total ordering) and underlying type

which translates to real C++20 concepts as:

template
  concept Movable = is_object_v && MoveConstructible && Assignable
    && Swappable;
template
  concept Copyable = CopyConstructible && Movable && Assignable;
template
  concept Semiregular = Copyable && DefaultConstructible;
template
  concept Regular = Semiregular && EqualityComparable;

We've lost the total ordering part in favor of simply EqualityComparable, and even then a lot of the library requirements via Ranges actually only require Semiregular - not Regular. But still, this is the foundation of the idea.

Note that if a type is movable, it already kind of makes sense for it to be default constructible. The moved-from state is very conceptually similar to a default-constructed state. Can't do much from there, but it's a state.