30 Input/output library [input.output]

30.9 File-based streams [file.streams]

30.9.5 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& rhs) = delete;
    basic_fstream(basic_fstream&& rhs);

    // [fstream.assign], assign and swap
    basic_fstream& operator=(const basic_fstream& rhs) = 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:
  • sb, the basic_­filebuf object.

30.9.5.1 basic_­fstream constructors [fstream.cons]

basic_fstream();
Effects: Constructs an object of class basic_­fstream<charT, traits>, initializing the base class with basic_­iostream<charT, traits>(&sb) ([iostream.cons]) and initializing sb with basic_­filebuf<charT, traits>().
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: Constructs an object of class basic_­fstream<charT, traits>, initializing the base class with basic_­iostream<charT, traits>(&sb) ([iostream.cons]) and initializing sb with basic_­filebuf<charT, traits>().
Then calls rdbuf()->open(s, mode).
If that function returns a null pointer, calls setstate(failbit).
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: The same as basic_­fstream(s.c_­str(), mode).
basic_fstream(basic_fstream&& 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.

30.9.5.2 Assign and swap [fstream.assign]

basic_fstream& operator=(basic_fstream&& rhs);
Effects: Move assigns the base and members of *this from the base and corresponding members of rhs.
Returns: *this.
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).
template <class charT, class traits> void swap(basic_fstream<charT, traits>& x, basic_fstream<charT, traits>& y);
Effects: As if by x.swap(y).

30.9.5.3 Member functions [fstream.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 | 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]).
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).
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]).