This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++17 status.
Section: 25.6.4.3 [istreambuf.iterator.cons] Status: C++17 Submitter: S. B. Tam Opened: 2015-10-05 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
N4527 25.6.4.3 [istreambuf.iterator.cons] does not mention what the effect of calling istreambuf_iterator(basic_streambuf<charT, traits>* s) is when s is a null pointer. It should be made clear that this case is well-formed and the result is a end-of-stream iterator.
Daniel: According to 25.6.4 [istreambuf.iterator] p1:[…] The default constructor istreambuf_iterator() and the constructor istreambuf_iterator(0) both construct an end-of-stream iterator object suitable for use as an end-of-range. […]
This indicates that the described constructor creates an end-of-stream iterator, but this wording is part of the introductory wording and I recommend to make 25.6.4.3 [istreambuf.iterator.cons] clearer, because the existing specification is already flawed, e.g. it never specifies when and how the exposition-only-member sbuf_ is initialized. The proposed wording below attempts to solve these problems as well.
Previous resolution [SUPERSEDED]:
This wording is relative to N4527.
Change 25.6.4.3 [istreambuf.iterator.cons] as indicated:
[Editorial note: The proposed wording changes also performs some editorial clean-up of the existing mismatches of the declarations in the class template synopsis and the individual member specifications. The below wording intentionally does not say anything about the concrete value of sbuf_ for end-of-stream iterator values, because that was never specified before; in theory, this could be some magic non-null pointer that can be used in constant expressions. But the wording could be drastically simplified by requiring sbuf_ to be a null pointer for an end-of-stream iterator value, since I have not yet seen any implementation where this requirement does not hold. — end editorial note]constexpr istreambuf_iterator() noexcept;-1- Effects: Constructs the end-of-stream iterator.
istreambuf_iterator(basic_istream<charT,traits>istream_type& s) noexcept;istreambuf_iterator(basic_streambuf<charT,traits>* s) noexcept;-2- Effects: If s.rdbuf() is a null pointer, constructs an end-of-stream iterator; otherwise initializes sbuf_ with s.rdbuf() and constructs an istreambuf_iterator that uses the streambuf_type object *sbuf_
Constructs an istreambuf_iterator<> that uses the basic_streambuf<> object *(s.rdbuf()), or *s, respectively. Constructs an end-of-stream iterator if s.rdbuf() is null.istreambuf_iterator(streambuf_type* s) noexcept;-?- Effects: If s is a null pointer, constructs an end-of-stream iterator; otherwise initializes sbuf_ with s and constructs an istreambuf_iterator that uses the streambuf_type object *sbuf_.
istreambuf_iterator(const proxy& p) noexcept;-3- Effects: Initializes sbuf_ with p.sbuf_ and constructs an istreambuf_iterator that uses the streambuf_type object *sbuf_
Constructs a istreambuf_iterator<> that uses the basic_streambuf<> object pointed to by the proxy object's constructor argument p.
[2015-10-20, Daniel provides alternative wording]
[2016-08-03 Chicago]
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 25.6.4.3 [istreambuf.iterator.cons] as indicated:
[Drafting note: The proposed wording changes also performs some editorial clean-up of the existing mismatches of the declarations in the class template synopsis and the individual member specifications. The below wording is simplified by requiring sbuf_ to be a null pointer for an end-of-stream iterator value, since I have not yet seen any implementation where this requirement does not hold. Even if there were such an implementation, this would still be conforming, because concrete exposition-only member values are not part of public API. — end drafting note]
For each istreambuf_iterator constructor in this section, an end-of-stream iterator is constructed if and only if the exposition-only member sbuf_ is initialized with a null pointer value.
constexpr istreambuf_iterator() noexcept;-1- Effects: Initializes sbuf_ with nullptr
Constructs the end-of-stream iterator.istreambuf_iterator(basic_istream<charT,traits>istream_type& s) noexcept;istreambuf_iterator(basic_streambuf<charT,traits>* s) noexcept;-2- Effects: Initializes sbuf_ with s.rdbuf()
Constructs an istreambuf_iterator<> that uses the basic_streambuf<> object *(s.rdbuf()), or *s, respectively. Constructs an end-of-stream iterator if s.rdbuf() is null.istreambuf_iterator(streambuf_type* s) noexcept;-?- Effects: Initializes sbuf_ with s.
istreambuf_iterator(const proxy& p) noexcept;-3- Effects: Initializes sbuf_ with p.sbuf_
Constructs a istreambuf_iterator<> that uses the basic_streambuf<> object pointed to by the proxy object's constructor argument p.