29 Input/output library [input.output]

29.9 File-based streams [file.streams]

29.9.5 Class template basic_fstream [fstream]

namespace std {
  template<class charT, class traits = char_traits<charT>>
  class basic_fstream : public basic_iostream<charT, traits> {
  public:
    using char_type   = charT;
    using int_type    = typename traits::int_type;
    using pos_type    = typename traits::pos_type;
    using off_type    = typename traits::off_type;
    using traits_type = traits;

    // [fstream.cons], constructors
    basic_fstream();
    explicit basic_fstream(
      const char* s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    explicit basic_fstream(
      const filesystem::path::value_type* s,
      ios_base::openmode mode = ios_base::in|ios_base::out);    // wide systems only; see [fstream.syn]
    explicit basic_fstream(
      const string& s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    explicit basic_fstream(
      const filesystem::path& s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    basic_fstream(const basic_fstream&) = delete;
    basic_fstream(basic_fstream&& rhs);

    // [fstream.assign], assign and swap
    basic_fstream& operator=(const basic_fstream&) = delete;
    basic_fstream& operator=(basic_fstream&& rhs);
    void swap(basic_fstream& rhs);

    // [fstream.members], members
    basic_filebuf<charT, traits>* rdbuf() const;
    bool is_open() const;
    void open(
      const char* s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    void open(
      const filesystem::path::value_type* s,
      ios_base::openmode mode = ios_base::in|ios_base::out);    // wide systems only; see [fstream.syn]
    void open(
      const string& s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    void open(
      const filesystem::path& s,
      ios_base::openmode mode = ios_base::in | ios_base::out);
    void close();

  private:
    basic_filebuf<charT, traits> sb;    // exposition only
  };

  template<class charT, class traits>
    void swap(basic_fstream<charT, traits>& x,
              basic_fstream<charT, traits>& y);
}

The class template basic_fstream<charT, traits> supports reading and writing from named files.

It uses a basic_filebuf<charT, traits> object to control the associated sequences.

For the sake of exposition, the maintained data is presented here as:

(1.1)
sb, the basic_filebuf object.

29.9.5.1 Constructors [fstream.cons]

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basic_fstream();

Effects: Initializes the base class with basic_iostream<charT, traits>(addressof(sb)) ([iostream.cons]) and sb with basic_filebuf<charT, traits>().

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explicit basic_fstream(
  const char* s,
  ios_base::openmode mode = ios_base::in | ios_base::out);
explicit basic_fstream(
  const filesystem::path::value_type* s,
  ios_base::openmode mode = ios_base::in | ios_base::out);  // wide systems only; see [fstream.syn]

Effects: Initializes the base class with basic_iostream<charT, traits>(addressof(sb)) ([iostream.cons]) and sb with basic_filebuf<charT, traits>().

Then calls rdbuf()->open(s, mode).

If that function returns a null pointer, calls setstate(failbit).

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explicit basic_fstream(
  const string& s,
  ios_base::openmode mode = ios_base::in | ios_base::out);
explicit basic_fstream(
  const filesystem::path& s,
  ios_base::openmode mode = ios_base::in | ios_base::out);

Effects: Equivalent to: basic_fstream(s.c_str(), mode).

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basic_fstream(basic_fstream&& rhs);

Effects: Move constructs the base class, and the contained basic_filebuf.

Then calls basic_istream<charT, traits>::set_rdbuf(addressof(sb)) to install the contained basic_filebuf.

29.9.5.2 Assignment and swap [fstream.assign]

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void swap(basic_fstream& rhs);

Effects: Exchanges the state of *this and rhs by calling basic_iostream<charT,traits>::swap(rhs) and sb.swap(rhs.sb).

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template<class charT, class traits>
  void swap(basic_fstream<charT, traits>& x,
            basic_fstream<charT, traits>& y);

Effects: Equivalent to: x.swap(y).

29.9.5.3 Member functions [fstream.members]

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basic_filebuf<charT, traits>* rdbuf() const;

Returns: const_cast<basic_filebuf<charT, traits>*>(addressof(sb)).

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bool is_open() const;

Returns: rdbuf()->is_open().

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void open(
  const char* s,
  ios_base::openmode mode = ios_base::in | ios_base::out);
void open(
  const filesystem::path::value_type* s,
  ios_base::openmode mode = ios_base::in | ios_base::out);  // wide systems only; see [fstream.syn]

Effects: Calls rdbuf()->open(s, mode).

If that function does not return a null pointer calls clear(), otherwise calls setstate(failbit) (which may throw ios_base::failure) ([iostate.flags]).

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void open(
  const string& s,
  ios_base::openmode mode = ios_base::in | ios_base::out);
void open(
  const filesystem::path& s,
  ios_base::openmode mode = ios_base::in | ios_base::out);

Effects: Calls open(s.c_str(), mode).

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void close();