27 Input/output library [input.output]

27.9 File-based streams [file.streams]

27.9.3 Class template basic_ifstream [ifstream]

namespace std {
  template<class charT, class traits = char_traits<charT>>
  class basic_ifstream : public basic_istream<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;

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

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

    // [ifstream.members], members
    basic_filebuf<charT, traits>* rdbuf() const;

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

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

The class basic_ifstream<charT, traits> supports reading from named files.

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

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

(1.1)
sb, the filebuf object.

27.9.3.1 basic_ifstream constructors [ifstream.cons]

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basic_ifstream();

Effects: Constructs an object of class basic_ifstream<charT, traits>, initializing the base class with basic_istream<charT, traits>(&sb) ([istream.cons]) and initializing sb with basic_filebuf<charT, traits>() ([filebuf.cons]).

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

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

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

Effects: The same as basic_ifstream(s.c_str(), mode).

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basic_ifstream(basic_ifstream&& rhs);

Effects: Move constructs from the rvalue rhs.

This is accomplished by move constructing the base class, and the contained basic_filebuf.

Next basic_istream<charT, traits>::set_rdbuf(&sb) is called to install the contained basic_filebuf.

27.9.3.2 Assign and swap [ifstream.assign]

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basic_ifstream& operator=(basic_ifstream&& rhs);

Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.

Returns: *this.

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void swap(basic_ifstream& rhs);

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

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

Effects: As if by x.swap(y).

27.9.3.3 Member functions [ifstream.members]

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

Returns: const_cast<basic_filebuf<charT, traits>*>(&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);
void open(const filesystem::path::value_type* s,
          ios_base::openmode mode = ios_base::in);  // wide systems only; see [fstream.syn]

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

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

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

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void close();

Effects: Calls rdbuf()->close() and, if that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure) ([iostate.flags]).