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NAME
ios - basic iostreams formatting
SYNOPSIS
#include <iostream.h>
class unsafe_ios {
public:
// exported types
// stream status bits
enum io_state {
goodbit = 0x00, // no bit set: all is ok
eofbit = 0x01, // at end of file
failbit = 0x02, // last I/O operation failed
badbit = 0x04, // invalid operation attempted
hardfail = 0x80 // unrecoverable error
};
// stream operation mode
enum open_mode {
in = 0x01, // open for reading
out = 0x02, // open for writing
ate = 0x04, // seek to eof upon original open
app = 0x08, // append mode: all additions at eof
trunc = 0x10, // truncate file if already exists
nocreate = 0x20, // open fails if file doesn't exist
noreplace= 0x40 // open fails if file already exists
};
// stream seek direction
enum seek_dir { beg=0, cur=1, end=2 };
// formatting flags
enum {
skipws = 0x0001, // skip whitespace on input
left = 0x0002, // left-adjust output
right = 0x0004, // right-adjust output
internal = 0x0008, // padding after sign or base indicator
dec = 0x0010, // decimal conversion
oct = 0x0020, // octal conversion
hex = 0x0040, // hexidecimal conversion
showbase = 0x0080, // use base indicator on output
showpoint = 0x0100, // force decimal point (floating output)
uppercase = 0x0200, // upper-case hex output
showpos = 0x0400, // add '+' to positive integers
scientific= 0x0800, // use 1.2345E2 floating notation
fixed = 0x1000, // use 123.45 floating notation
unitbuf = 0x2000, // flush all streams after insertion
stdio = 0x4000 // flush stdout, stderr after insertion
};
public:
// exported functions
// format-related functions
long flags();
long flags(long bits);
long setf(long setbits, long field);
long setf(long setbits);
long unsetf(long unsetbits);
int width();
int width(int len);
char fill();
char fill(char ch);
int precision(int len);
int precision();
int skip(int doskip); // obsolete
// state-related functions
int rdstate();
int eof();
int fail();
int bad();
int good();
void clear(int state=0);
// other functions
ostream* tie(ostream* tiedstream);
ostream* tie();
streambuf* rdbuf();
static long bitalloc();
static int xalloc();
long & iword(int index);
void* & pword(int index);
static void sync_with_stdio();
public:
// exported operator and conversion functions
operator void* ();
int operator ! ();
public:
// exported constructor, destructor
unsafe_ios(streambuf* sbp);
virtual ~unsafe_ios();
public:
// exported data members
static const long basefield; // dec | oct | hex
static const long adjustfield; // left | right | internal
static const long floatfield; // scientific | fixed
protected:
// protected function
void init(streambuf* sbp);
// protected constructor
unsafe_ios();
void setstate (int);
static void (*stdioflush)();
private:
// private members to prevent copying
unsafe_ios(unsafe_ios&);
void operator= (unsafe_ios&);
};
class ios : virtual public unsafe_ios, public stream_MT {
public:
// format-related functions
long flags();
long flags(long);
long setf(long setbits, long field);
long setf(long);
long unsetf(long);
int width();
int width(int);
char fill();
char fill(char);
int precision();
int precision(int);
// state-related functions
int rdstate();
int eof();
int fail();
int bad();
int good();
void clear(int state =0);
// other functions
ostream* tie();
ostream* tie(ostream*);
streambuf* rdbuf();
static long bitalloc();
static int xalloc();
long& iword(int);
void* & pword(int);
static void sync_with_stdio();
public:
// exported operator and conversion functions
operator void*();
int operator!();
public:
// exported operator and conversion functions
ios(streambuf* sbp);
virtual ~ios();
protected:
init(streambuf* sbp);
ios();
setstate(int);
static void (*stdioflush)();
protected:
static stream_rmutex static_mutlock;
static int mutex_init_count;
private:
ios(ios&);
void operator=(ios&) ;
};
// Predefined manipulators
unsafe_ostream& endl(unsafe_ostream&);
unsafe_ostream& ends(unsafe_ostream&);
unsafe_ostream& flush(unsafe_ostream&);
ostream& endl(ostream&);
ostream& ends(ostream&);
ostream& flush(ostream&);
unsafe_ios& dec(unsafe_ios&);
unsafe_ios& hex(unsafe_ios&);
unsafe_ios& oct(unsafe_ios&);
ios& dec(ios&);
ios& hex(ios&);
ios& oct(ios&);
unsafe_istream& ws(unsafe_istream&);
istream& ws(istream&);
DESCRIPTION
Class ios is a virtual base class of all stream objects. It
provides the basic state and formatting data for a stream.
Several enumerations are defined, and a large collection of
functions. These are described below.
Class unsafe_ios implements all of the functionality of this
class. Class ios is a "wrapper" class that implements mutex
locks around each of the member functions of unsafe_ios to
protect against access by multiple threads. Use of the pro-
tected class does not guarantee mt-safety, however; for more
information on making your application mt-safe, see the C++
4.1 Library Reference Manual, Chapter 5, "Using libC in a
Multithreaded Environment."
ENVIRONMENT
The discussion about thread safety applies only to Solaris
2.x. On Solaris 1.x the ``safe'' and ``unsafe'' functions
behave identically.
Enumerations
io_state
Member functions use these enumerations to keep track
of the error state of the stream. See also the ``Error
States'' section below for how to test these bits.
io_state is really a collection of bits, as follows:
goodbit
This ``bit'' is really the absence of any error
bits, and indicates that the stream is in a good
state.
eofbit
This bit is normally set when end of file has been
reached during an extraction. It is not set as
the result of a succesful extraction reaching end
of file, but when end of file is reached while
attempting further extractions. ``End of file''
in this sense is an abstraction as defined by the
streambuf associated with the stream. Normally
this bit is not set for output streams.
failbit
This bit is set when an attempted extraction or
conversion has failed, usually due to unexpected
characters. Further attempted extractions will
fail until this bit is cleared, to prevent running
on after improper input. Usually the stream is
still usable, and extraction may be continued
after clearing the bit and dealing with the unex-
pected input.
badbit
This bit indicates that some operation on the
associated streambuf has failed. Typically,
attempting further operations will not succeed,
even after clearing the bit. Example situations
would be an output error, or immediate end of file
on an attempted input operation.
hardfail
This bit is reserved to the implementation to
indicate that the stream cannot be further used.
Typically it represents a hardware failure of some
kind. The bit cannot be cleared by any publicly-
accessible function.
open_mode
These enumerations are described in fstream(3C++),
under the description of function open().
seek_dir
These enumerations are described in sbufpub(3C++),
under the description of function seekoff().
formatting flags
Member functions use these enumerations of anonymous
type to control input and output formatting. See the
"Format Control" section below.
Constructors and Assignment
ios(sbp)
The streambuf pointed to by sbp becomes the streambuf
associated with the ios being constructed. The pointer
must not be null.
ios() // protected
init(sbp) // protected
Historically, a virtual base class required a default
constructor (one with no arguments), because there used
to be no way to pass arguments to a constructor for a
virtual base class. Class ios therefore has a default
constructor and a separate intialization function tak-
ing a pointer to a streambuf. A derived class uses the
protected constructor ios() by default, and calls ini-
tialization function init(streambuf*). The argument to
init points to the streambuf to be associated with the
ios being constructed, and must not be null. Example:
class istream : virtual public ios { ... };
istream::istream(streambuf* s)
{
ios::init(s);
// ...
}
ios(iosref) // private
stream2=stream1 // private
The copy constructor and assignment operator are
private to prevent copying of ios objects, since the
effect of such copying is not well defined. Usually
you want to copy a pointer to the object, or pass a
reference to a function.
Error States
Several functions enable testing and adjusting the error
state bits, as follows.
int i = s.rdstate()
Returns the error state bits of stream s as an int.
s.clear(state)
Stores its int parameter as the error state of stream
s. The value of state should be derived only from the
return of rdstate and/or combinations of the io_state
bits. To clear only one bit in the stream state, use
something like s.clear(~ios::failbit & s.rdstate());
int i = s.good()
Returns non-zero if the error state is good; that is,
if no bits are set. Otherwise, returns zero. In par-
ticular, returns zero if eofbit is set.
int i = s.eof()
Returns non-zero if the eofbit is set, zero otherwise.
int i = s.fail()
Returns non-zero if any of failbit, badbit, or hardfail
is set, zero otherwise.
int i = s.bad()
Returns non-zero if either of badbit or hardfail is
set, zero otherwise.
Other Status Testing
It is often convenient to be able to test the state of a
stream directly. Since typical insertion and extraction
operators return a reference to the stream, you can test the
return values of the operations. Two operators are defined
to permit this testing.
operator void* ()
You may use an explict cast of a stream to void*, or
use a stream in a boolean context to test its state.
The result is 0 if any of failbit, badbit, or hardfail
is set. The result is a non-zero pointer if the stream
is in a good or eof state. Examples:
if( cout ) ... // next output will probably succeed
if( cin >> x ) ... // input to x succeeded
operator ! ()
This operator provides the inverse of the above test-
ing. The result is non-zero if any of failbit, badbit,
or hardfail is set, zero otherwise. Examples:
if( ! cout ) ... // output will not succeed
if( ! (cin >> x) ) ... // input to x failed
Format Control
A ios maintains a format state which is controlled by for-
matting flags, and the three functions fill(), width(), and
precision(). The formatting flags are a collection of bits
described below, declared as enumerations of an anonymous
type. These format state bits are kept in a long int and
may be manipulated independently via two versions of the
flags() function.
The formatting flags may be set and cleared independent of
other operations. They change only by explicit programmer
action. The flags are as follows:
skipws
If this flag is set, formatted extractors will skip
leading whitespace; otherwise, leading whitespace is
not skipped. This flag is set by default, allowing
free-format input text. Unformatted extractors do not
examine this flag.
left
right
internal
These flags control how padding is inserted during for-
matted operations. At most one of these three flags
may be set at one time. The three flags may be
addressed as a unit by the static member
ios::adjustfield. If left is set, the value is left-
adjusted in its field width, meaning that padding is
added on the right. If right is set, the value is
right-adjusted in its field width, meaning that padding
is added on the left. If internal is set, padding is
added after any leading base or sign field, and before
the value. The default (none of the flags set) is
right. The fill character used for padding defaults to
the space character, and may be set with the fill func-
tion. The amount of padding is determined by the field
width as set by the width function. See also
manip(3C++).
dec
oct
hex
These flags control the conversion base of integer
data. At most one of these three flags may be set at
one time. The three flags may be addressed as a unit
by the static member ios::basefield. Conversions are
done in decimal (base 10) if dec is set, in octal (base
8) if oct is set, or in hexadecimal (base 16) if hex is
set. If none of the flags is set, insertions are done
in decimal, and extractions are converted according to
the C++ rules for representing integer constants. That
is, a leading ``0x'' or ``0X'' will result in hex
conversion, a leading `0' will result in octal conver-
sion, and a leading `1' through `9' will result in
decimal conversion for extraction. The default is none
of these bits set. The manipulators dec, oct, and hex
may also be used to set the conversion base as
described below in section ``Predefined Manipulators''.
showbase
If this flag is set, insertions of converted integral
values will be in the form used for representing C++
integer constants. That is, octal values will begin
with a leading `0', and hexadecimal values will begin
with a leading ``0x'' or ``0X''. (See ``uppercase''
below.) The default is unset.
showpos
If this flag is set, a plus sign (`+') will be added to
insertions of converted positive decimal values
(including floating-point). The default is unset.
uppercase
If this flag is set, an uppercase `X' will be used in
insertions of converted hexidecimal values when
showbase is set, and an uppercase `E' will be used for
floating-point conversions. Otherwise, lowercase `x'
and `e' will be used, respectively. The default is
unset.
fixed
scientific
These flags control the type of conversion used when
floating-point values are converted for insertion. The
two flags may be addressed as a unit by the static
member ios::floatfield. The rules followed for conver-
sion are generally the same as for the C stdio function
printf. (See printf(3V).) If scientific is set, `e'
format is used. If fixed is set, `f' format is used.
If neither is set, `g' format is used. (See also
uppercase above.) The value set by width, if any, is
used as the printf field width specification. The
value set by precision, if any, is used as the printf
precision specification.
showpoint
If this flag is set, trailing zeros, or a trailing
decimal point, will appear in the conversion of
floating-point values. The default is to truncate
trailing zeros or a trailing decimal point.
unitbuf
If an output stream is buffered, the buffer is flushed
when it fills, or when it is explicitly flushed. This
can result in delayed output, or lost output if the
program should crash. A stream may be unbuffered,
eliminating delays and lost output, but at the cost of
a system call per character output. If the unitbuf is
set, the buffer will be flushed after each complete
insertion. Unit buffering is thus a compromise, pro-
viding frequent output at lower cost than unbuffered
output, and not requiring extra flush calls in the pro-
gram source. In particular, unit buffering may be
turned on and off at selected places in the code
without changing any other source code. By default,
this flag is not set.
stdio
This flag causes the C stdio files stdout and stderr to
be flushed after each insertion in the stream. This
may be useful when C stdio on the standard files is
mixed with iostreams on other files. By default, this
flag is not set.
The format control functions are as follows:
long theflags = s.flags()
Returns the current formatting flags of stream s in a
long.
long oldflags = s.flags(newflags)
Uses the long value of newflags to replace all the
formatting flags in stream s. Returns the previous
formatting flags in a long.
long oldflags = s.setf(newflags)
Each bit which is set in the long value newflags is set
in the formatting flags of stream s. The remaining
formatting flags are unaffected. Returns the previous
formatting flags in a long. Note the flags function
replaces all the flag bits, while the setf function
sets just those bits which are specified. This version
of setf is most useful for setting a flag which is not
part of a group. See the second form of this function
below for setting one of a group of flags.
long oldflags = s.setf(newflags, field)
The bits which are set in the long value field mark the
formatting flags which are replaced by the correspond-
ing bits in the long value newflags. Returns the pre-
vious value of the designated flags. Typically, one of
the constants basefield, adjustfield, or floatfield is
used as the value of field. Example - set to left-
justification, output a value, and restore the previous
justification:
long oldadjust = cout.setf(ios::left, ios::adjustfield);
cout << data;
cout.setf(oldadjust, ios::adjustfield);
This technique ensures that only one of the adjustfield
bits is ever set, and allows convenient restoration of
the previous status. Using zero for the new value of
the field will clear just those flags. Example - clear
the integer conversion base to the default state:
cout.setf(0, ios::basefield);
See also the manipulators setiosflags and resetiosflags
in manip(3C++).
long oldflags = s.unsetf(newflags)
Each bit which is set in the long value newflags is
unset in the formatting flags of stream s. The remain-
ing formatting flags are unaffected. Returns the pre-
vious formatting flags in a long. Note the setf func-
tion sets corresponding flag bits, while the unsetf
function clears them. See also the manipulator
resetiosflags in manip(3C++).
char thefill = s.fill()
Returns the current fill character of stream s. The
fill character is used for padding an insertion to the
designated field width. This release supports only
single-byte characters for fill. See the discussion
above for left, right, and internal.
char oldfill = s.fill(newfill)
Sets the fill character of stream s to newfill and
returns the old fill character. The default fill char-
acter is the space. See also the manipulator setfill
in manip(3C++).
int theprec = s.precision()
Returns the current ``precision'' format state of
stream s. It controls the number of significant digits
converted in floating-point insertions. See the dis-
cussion above for scientific and fixed.
int oldprec = s.precision(newprec)
Sets the ``precision'' format state of stream s to
newprec, and returns the old value. The default value
is 6. See also the manipulator setprecision in
manip(3C++).
int thewidth = s.width()
Returns the current ``field width'' format state of
stream s. If the field width is zero, inserters will
insert only the characters necessary to represent the
value being inserted. If the field width is greater
than the number of characters needed, the field will be
padded with the fill character to the specified width,
as described above. If the field width is less than
the number of characters needed, the width will be
extended. The field width represents the minimum field
width; it cannot be used to provide truncation to a
maximum field width. For wide character output the
width is still measured in characters, not in bytes.
int oldwidth = s.width(newwidth)
Sets the ``field width'' format state of stream s to
newwidth, and returns the old value. The default value
is 0. The field width is reset to zero automatically
after every formatted insertion or extraction. It must
therefore be reset for each operation requiring a field
width. See also the manipulator setw in manip(3C++).
User-defined Format Flags
User-defined format flags and variables are provided for
derived classes which may need their own. Once allocated
for a class, the flags and variables are reserved for the
duration of the program; several independent classes may
allocate their own flags and variables without conflict.
long thebit = ios::bitalloc()
This static member function returns a long with one
previously unallocated flag bit set. This value may
then be used as a flag, for example, in calls to setf,
for class-specific purposes. At least 16 bits are
available for allocation. When no bits are available,
this function returns zero.
int index = ios::xalloc()
This static member function returns a previously unused
index into an array of words. A word is big enough to
contain a long or a void*. This index may then be used
with functions iword or ipword to get a reference to a
reserved status variable.
long theword = s.iword(i)
void* theptr = s.ipword(i)
When i is an index value returned by a call to
ios::xalloc, these functions return a reference to the
ith user-defined status variable (word) for class s.
Function iword returns the reference typed as a long,
and function ipword returns the reference typed as a
void*. Note: Do not depend on the returned reference
being stable for an indefinite period. In particular,
any call to xalloc() may invalidate a previous refer-
ence.
Miscellaneous Functions
streambuf* sbp = s.rdbuf()
Returns a pointer to the streambuf associated with the
stream. This is part of the construction of a stream,
and the buffer class object is not normally changed.
This function may be used to get at streambuf functions
directly, given a stream object.
ostream* oldosp = s.tie(osp)
A stream may be ``tied'' to one ostream, kept track of
by the ``tie'' stream variable. Whenever a stream
needs to acquire more input or flush its output, the
tied stream, if any, is flushed first. For example,
cin is initially tied to cout, so that pending output,
such as a prompt, will be flushed before new input is
attempted. This function sets the tie variable of
stream s to the ostream pointed to by input parameter
osp, and returns the old value of the tie variable.
The sequence
ostream* oldosp = s.tie(0);
... do something ...
s.tie(oldosp);
will untie a stream while some work is done, then
restore the previous tie.
ostream* theosp = s.tie()
Returns the current value of the ``tie'' variable.
(See above.)
ios::sync_with_stdio()
If C stdio and C++ stream operations are performed on
the same standard file, synchronization problems will
occur. Since each style of I/O will have its own
buffering, I/O will not occur in the order of program
execution. To solve this synchronization problem, call
this static function prior to doing any I/O to any of
the standard streams cin, cout, cerr, or clog. This
function resets the standard streams to use stdiobufs.
I/O via stdio and streams will then be synchronized.
There is a substantial performance degradation compared
to using just buffered stream I/O or just buffered
stdio. See stdiobuf(3C++). Note: sync_with_stdio is
needed only when doing I/O to the same standard input,
output, or error file. You may use exclusively stdio
input functions on stdin and exclusively stream output
functions on cout with no difficulty.
Predefined Manipulators
A manipulator may be used apparently as an inserted or
extracted object, but really only changes the state of the
stream. See manip(3C++) for more information. Several
manipulators are predefined for use with streams.
s >> dec
s << dec
These set the conversion base of stream s to 10.
s >> oct
s << oct
These set the conversion base of stream s to 8.
s >> hex
s << hex
These set the conversion base of stream s to 16.
s >> ws
This extracts and discards whitespace from stream s.
See istream(3C++).
s << endl
Inserts a newline into stream s and flushes the stream.
See ostream(3C++).
s << ends
Inserts a null character (`\0') into stream s to end
the string. See strstream(3C++).
s << flush
Flushes the stream s. See ostream(3C++).
Additional manipulators become available when you include
<manip.h>. See manip(3C++)
SEE ALSO
ios.intro(3C++), filebuf(3C++), fstream(3C++),
istream(3C++), manip(3C++), ostream(3C++), printf(3V)
sbufprot(3C++), sbufpub(3C++), ssbuf(3C++), stdiobuf(3C++),
strstream(3C++), stream_locker(3C++), stream_MT(3C++),
C++ 4.1 Library Reference Manual:
Chapter 4, "The Iostream Library",
Chapter 5, "Using libC in a Multithreaded Environment."
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