23 Iterators library [iterators]

23.6 Stream iterators [stream.iterators]

23.6.3 Class template istreambuf_­iterator [istreambuf.iterator]

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The class template istreambuf_­iterator defines an input iterator that reads successive characters from the streambuf for which it was constructed.
operator* provides access to the current input character, if any.
Each time operator++ is evaluated, the iterator advances to the next input character.
If the end of stream is reached (streambuf_­type​::​sgetc() returns traits​::​eof()), the iterator becomes equal to the end-of-stream iterator value.
The default constructor istreambuf_­iterator() and the constructor istreambuf_­iterator(nullptr) both construct an end-of-stream iterator object suitable for use as an end-of-range.
All specializations of istreambuf_­iterator shall have a trivial copy constructor, a constexpr default constructor, and a trivial destructor.
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The result of operator*() on an end-of-stream iterator is undefined.
For any other iterator value a char_­type value is returned.
It is impossible to assign a character via an input iterator.
namespace std {
  template<class charT, class traits = char_traits<charT>>
  class istreambuf_iterator {
  public:
    using iterator_category = input_iterator_tag;
    using value_type        = charT;
    using difference_type   = typename traits::off_type;
    using pointer           = unspecified;
    using reference         = charT;
    using char_type         = charT;
    using traits_type       = traits;
    using int_type          = typename traits::int_type;
    using streambuf_type    = basic_streambuf<charT,traits>;
    using istream_type      = basic_istream<charT,traits>;

    class proxy;                          // exposition only

    constexpr istreambuf_iterator() noexcept;
    constexpr istreambuf_iterator(default_sentinel_t) noexcept;
    istreambuf_iterator(const istreambuf_iterator&) noexcept = default;
    ~istreambuf_iterator() = default;
    istreambuf_iterator(istream_type& s) noexcept;
    istreambuf_iterator(streambuf_type* s) noexcept;
    istreambuf_iterator(const proxy& p) noexcept;
    istreambuf_iterator& operator=(const istreambuf_iterator&) noexcept = default;
    charT operator*() const;
    istreambuf_iterator& operator++();
    proxy operator++(int);
    bool equal(const istreambuf_iterator& b) const;

    friend bool operator==(const istreambuf_iterator& i, default_sentinel_t s);

  private:
    streambuf_type* sbuf_;              // exposition only
  };
}

23.6.3.1 Class istreambuf_­iterator​::​proxy [istreambuf.iterator.proxy]

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Class istreambuf_­iterator<charT,traits>​::​proxy is for exposition only.
An implementation is permitted to provide equivalent functionality without providing a class with this name.
Class istreambuf_­iterator<charT, traits>​::​proxy provides a temporary placeholder as the return value of the post-increment operator (operator++).
It keeps the character pointed to by the previous value of the iterator for some possible future access to get the character.
namespace std {
  template<class charT, class traits>
  class istreambuf_iterator<charT, traits>::proxy { // exposition only
    charT keep_;
    basic_streambuf<charT,traits>* sbuf_;
    proxy(charT c, basic_streambuf<charT,traits>* sbuf)
      : keep_(c), sbuf_(sbuf) { }
  public:
    charT operator*() { return keep_; }
  };
}

23.6.3.2 Constructors [istreambuf.iterator.cons]

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For each istreambuf_­iterator constructor in this subclause, an end-of-stream iterator is constructed if and only if the exposition-only member sbuf_­ is initialized with a null pointer value.
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constexpr istreambuf_iterator() noexcept; constexpr istreambuf_iterator(default_sentinel_t) noexcept;
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Effects: Initializes sbuf_­ with nullptr.
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istreambuf_iterator(istream_type& s) noexcept;
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Effects: Initializes sbuf_­ with s.rdbuf().
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istreambuf_iterator(streambuf_type* s) noexcept;
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Effects: Initializes sbuf_­ with s.
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istreambuf_iterator(const proxy& p) noexcept;
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Effects: Initializes sbuf_­ with p.sbuf_­.

23.6.3.3 Operations [istreambuf.iterator.ops]

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charT operator*() const;
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Returns: The character obtained via the streambuf member sbuf_­->sgetc().
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istreambuf_iterator& operator++();
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Effects: As if by sbuf_­->sbumpc().
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Returns: *this.
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proxy operator++(int);
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Returns: proxy(sbuf_­->sbumpc(), sbuf_­).
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bool equal(const istreambuf_iterator& b) const;
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Returns: true if and only if both iterators are at end-of-stream, or neither is at end-of-stream, regardless of what streambuf object they use.
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template<class charT, class traits> bool operator==(const istreambuf_iterator<charT,traits>& a, const istreambuf_iterator<charT,traits>& b);
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Returns: a.equal(b).
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friend bool operator==(const istreambuf_iterator& i, default_sentinel_t s);
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Returns: i.equal(s).