27 Input/output library [input.output]

27.5 Iostreams base classes [iostreams.base]

27.5.3 Class ios_base [ios.base]

27.5.3.1 Types [ios.types]

27.5.3.1.1 Class ios_base::failure [ios::failure]

namespace std {
  class ios_base::failure : public system_error {
  public:
    explicit failure(const string& msg, const error_code& ec = io_errc::stream);
    explicit failure(const char* msg, const error_code& ec = io_errc::stream);
  };
}
1

The class failure defines the base class for the types of all objects thrown as exceptions, by functions in the iostreams library, to report errors detected during stream buffer operations.

2

When throwing ios_base::failure exceptions, implementations should provide values of ec that identify the specific reason for the failure. [ Note: Errors arising from the operating system would typically be reported as system_category() errors with an error value of the error number reported by the operating system. Errors arising from within the stream library would typically be reported as error_code(io_errc::stream, iostream_category()).  — end note ]

explicit failure(const string& msg, const error_code& ec = io_errc::stream);

3

Effects: Constructs an object of class failure by constructing the base class with msg and ec.

explicit failure(const char* msg, const error_code& ec = io_errc::stream);

4

Effects: Constructs an object of class failure by constructing the base class with msg and ec.

27.5.3.1.2 Type ios_base::fmtflags [ios::fmtflags]

typedef T1 fmtflags;

1

The type fmtflags is a bitmask type ([bitmask.types]). Setting its elements has the effects indicated in Table [tab:iostreams.fmtflags.effects].

Table 122 — fmtflags effects
ElementEffect(s) if set
boolalpha insert and extract bool type in alphabetic format
dec converts integer input or generates integer output in decimal base
fixed generate floating-point output in fixed-point notation
hex converts integer input or generates integer output in hexadecimal base
internal adds fill characters at a designated internal point in certain generated output, or identical to right if no such point is designated
left adds fill characters on the right (final positions) of certain generated output
oct converts integer input or generates integer output in octal base
right adds fill characters on the left (initial positions) of certain generated output
scientific generates floating-point output in scientific notation
showbase generates a prefix indicating the numeric base of generated integer output
showpoint generates a decimal-point character unconditionally in generated floating-point output
showpos generates a + sign in non-negative generated numeric output
skipws skips leading whitespace before certain input operations
unitbuf flushes output after each output operation
uppercase replaces certain lowercase letters with their uppercase equivalents in generated output
2

Type fmtflags also defines the constants indicated in Table [tab:iostreams.fmtflags.constants].

Table 123 — fmtflags constants
ConstantAllowable values
adjustfield left | right | internal
basefield dec | oct | hex
floatfield scientific | fixed

27.5.3.1.3 Type ios_base::iostate [ios::iostate]

typedef T2 iostate;

1

The type iostate is a bitmask type ([bitmask.types]) that contains the elements indicated in Table [tab:iostreams.iostate.effects].

Table 124 — iostate effects
ElementEffect(s) if set
badbit indicates a loss of integrity in an input or output sequence (such as an irrecoverable read error from a file);
eofbit indicates that an input operation reached the end of an input sequence;
failbit indicates that an input operation failed to read the expected characters, or that an output operation failed to generate the desired characters.
2

Type iostate also defines the constant:

  • (2.1)

    goodbit, the value zero.

27.5.3.1.4 Type ios_base::openmode [ios::openmode]

typedef T3 openmode;

1

The type openmode is a bitmask type ([bitmask.types]). It contains the elements indicated in Table [tab:iostreams.openmode.effects].

Table 125 — openmode effects
ElementEffect(s) if set
app seek to end before each write
ate open and seek to end immediately after opening
binary perform input and output in binary mode (as opposed to text mode)
in open for input
out open for output
trunc truncate an existing stream when opening

27.5.3.1.5 Type ios_base::seekdir [ios::seekdir]

typedef T4 seekdir;

1

The type seekdir is an enumerated type ([enumerated.types]) that contains the elements indicated in Table [tab:iostreams.seekdir.effects].

Table 126 — seekdir effects
ElementMeaning
beg request a seek (for subsequent input or output) relative to the beginning of the stream
cur request a seek relative to the current position within the sequence
end request a seek relative to the current end of the sequence

27.5.3.1.6 Class ios_base::Init [ios::Init]

namespace std {
  class ios_base::Init {
  public:
    Init();
   ~Init();
  private:
    static int init_cnt; // exposition only
  };
}
1

The class Init describes an object whose construction ensures the construction of the eight objects declared in <iostream> ([iostream.objects]) that associate file stream buffers with the standard C streams provided for by the functions declared in <cstdio> ([c.files]).

2

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

  • (2.1)

    static int init_cnt, counts the number of constructor and destructor calls for class Init, initialized to zero.

Init();

3

Effects: Constructs an object of class Init. Constructs and initializes the objects cin, cout, cerr, clog, wcin, wcout, wcerr, and wclog if they have not already been constructed and initialized.

~Init();

4

Effects: Destroys an object of class Init. If there are no other instances of the class still in existence, calls cout.flush(), cerr.flush(), clog.flush(), wcout.flush(), wcerr.flush(), wclog.flush().