30 Input/output library [input.output]

30.9 File-based streams [file.streams]

30.9.3 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:

30.9.3.1 basic_­ifstream constructors [ifstream.cons]

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]).
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]
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]), then calls rdbuf()->open(s, mode | ios_­base​::​in).
If that function returns a null pointer, calls setstate(failbit).
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).
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.

30.9.3.2 Assign and swap [ifstream.assign]

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.
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).
template <class charT, class traits> void swap(basic_ifstream<charT, traits>& x, basic_ifstream<charT, traits>& y);
Effects: As if by x.swap(y).

30.9.3.3 Member functions [ifstream.members]

basic_filebuf<charT, traits>* rdbuf() const;
Returns: const_­cast<basic_­filebuf<charT, traits>*>(&sb).
bool is_open() const;
Returns: rdbuf()->is_­open().
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]).
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).
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]).