c++ - 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), ranges::ostream_iterator is default-constructible - without an ostream.

How come?

I thought that "dummy" construction with effective construction later is an anti-pattern in C++, a wart we are gradually getting rid of. std::ostream iterator can only be constructed with a stream (for now - before C++20). And it's not as though we can do anything with the default-constructed range::ostream_iterator... So, what's the deal?

Answer 1

As an update to this, P2325R3 was just adopted which makes std::ostream_iterator no longer default constructible (it was briefly made so in C++20).

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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<class T>
  concept Movable = is_object_v<T> && MoveConstructible<T> && Assignable<T&, T>
    && Swappable<T>;
template<class T>
  concept Copyable = CopyConstructible<T> && Movable<T> && Assignable<T&, const T&>;
template<class T>
  concept Semiregular = Copyable<T> && DefaultConstructible<T>;
template<class T>
  concept Regular = Semiregular<T> && EqualityComparable<T>;

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.