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### 2878. Missing `DefaultConstructible` requirement for `istream_iterator` default constructor

Section: 25.6.2.2 [istream.iterator.cons] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 153

`istream_iterator` default constructor requires a `DefaultConstructible T`.

Proposed change:

Add a new p1:

> Requires: `T` is `DefaultConstructible`.

Previous resolution [SUPERSEDED]:

This wording is relative to N4618.

1. Modify 25.6.2.2 [istream.iterator.cons] as indicated:

   >     see below istream_iterator();

   > > -?- Requires: `T` is `DefaultConstructible`.

   > > -1- Effects: Constructs the end-of-stream iterator. If `is_trivially_default_constructible_v<T>` is `true`, then this constructor is a `constexpr` constructor.

   > > -2- Postconditions: `in_stream == 0`.

[Kona 2017-02-28: Jonathan provides updated wording as requested by LWG.]

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4618.

1. Modify 25.6.2 [istream.iterator] as indicated:

   > -1- The class template `istream_iterator` is an input iterator (24.2.3) that reads (using `operator>>`) successive elements from the input stream for which it was constructed. After it is constructed, and every time `++` is used, the iterator reads and stores a value of `T`. If the iterator fails to read and store a value of `T` (`fail()` on the stream returns `true`), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments `istream_iterator()` always constructs an end-of-stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of `operator*` on an end-of-stream iterator is not defined. For any other iterator value a `const T&` is returned. The result of `operator->` on an end-of-stream iterator is not defined. For any other iterator value a `const T*` is returned. The behavior of a program that applies `operator++()` to an end-of-stream iterator is undefined. It is impossible to store things into istream iterators. The type `T` shall meet the `DefaultConstructible`, `CopyConstructible`, and `CopyAssignable` requirements.