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The previous chapter referred in passing to the classes ostream
and istream, for output and input respectively. These classes
share certain properties, captured in their base class ios.
ios
The base class ios provides methods to test and manage the state
of input or output streams.
ios delegates the job of actually reading and writing bytes to
the abstract class streambuf, which is designed to provide
buffered streams (compatible with C, in the GNU implementation).
See section Using the streambuf Layer, for information on
the facilities available at the streambuf level.
Constructor: ios::ios ([streambuf* sb [, ostream* tie])
The ios constructor by default initializes a new ios, and
if you supply a streambuf sb to associate with it, sets the
state good in the new ios object. It also sets the
default properties of the new object.
You can also supply an optional second argument tie to the
constructor: if present, it is an initial value for ios::tie, to
associate the new ios object with another stream.
The ios destructor is virtual, permitting application-specific
behavior when a stream is closed--typically, the destructor frees any
storage associated with the stream and releases any other associated
objects.
Use this collection of methods to test for (or signal) errors and other exceptional conditions of streams:
Method: ios::operator void* () const
You can do a quick check on the state of the most recent operation on a
stream by examining a pointer to the stream itself. The pointer is
arbitrary except for its truth value; it is true if no failures have
occurred (ios::fail is not true). For example, you might ask for
input on cin only if all prior output operations succeeded:
if (cout)
{
// Everything OK so far
cin >> new_value;
...
}
Method: ios::operator ! () const
In case it is more convenient to check whether something has failed, the
operator ! returns true if ios::fail is true (an operation
has failed). For example,
you might issue an error message if input failed:
if (!cin)
{
// Oops
cerr << "Eh?\n";
}
Method: iostate ios::rdstate () const
Return the state flags for this stream. The value is from the
enumeration iostate. You can test for any combination of
goodbit
eofbit
failbit
badbit
Method: void ios::setstate (iostate state)
Set the state flag for this stream to state in addition to
any state flags already set. Synonym (for upward compatibility):
ios::set.
See ios::clear to set the stream state without regard to existing
state flags. See ios::good, ios::eof, ios::fail,
and ios::bad, to test the state.
Method: int ios::good () const
Test the state flags associated with this stream; true if no error indicators are set.
Test whether a stream is marked as unusable. (Whether
ios::badbit is set.)
True if end of file was reached on this stream. (If ios::eofbit
is set.)
Method: int ios::fail () const
Test for any kind of failure on this stream: either some
operation failed, or the stream is marked as bad. (If either
ios::failbit or ios::badbit is set.)
Method: void ios::clear (iostate state)
Set the state indication for this stream to the argument state.
You may call ios::clear with no argument, in which case the state
is set to good (no errors pending).
See ios::good, ios::eof, ios::fail, and
ios::bad, to test the state; see ios::set or
ios::setstate for an alternative way of setting the state.
These methods control (or report on) settings for some details of controlling streams, primarily to do with formatting output:
Method: char ios::fill () const
Report on the padding character in use.
Method: char ios::fill (char padding)
Set the padding character. You can also use the manipulator
setfill. See section Changing stream properties using manipulators.
Default: blank.
Method: int ios::precision () const
Report the number of significant digits currently in use for output of floating point numbers.
Default: 6.
Method: int ios::precision (int signif)
Set the number of significant digits (for input and output numeric conversions) to signif.
You can also use the manipulator setprecision for this purpose.
See section Changing stream properties using manipulators.
Method: int ios::width () const
Report the current output field width setting (the number of characters to write on the next `<<' output operation).
Default: 0, which means to use as many characters as necessary.
Method: int ios::width (int num)
Set the input field width setting to num. Return the previous value for this stream.
This value resets to zero (the default) every time you use `<<'; it is
essentially an additional implicit argument to that operator. You can
also use the manipulator setw for this purpose.
See section Changing stream properties using manipulators.
Method: fmtflags ios::flags () const
Return the current value of the complete collection of flags controlling the format state. These are the flags and their meanings when set:
ios::dec
ios::oct
ios::hex
setbase, or any of the manipulators dec, oct, or
hex; see section Changing stream properties using manipulators.)
On input, if none of these flags is set, read numeric constants according to the prefix: decimal if no prefix (or a `.' suffix), octal if a `0' prefix is present, hexadecimal if a `0x' prefix is present.
Default: dec.
ios::fixed
ios::precision to set precision.
ios::left
ios::right
ios::internal
ios::scientific
ios::showbase
ios::showpoint
ios::showpos
ios::skipws
ios::stdio
stdio streams stdout and stderr after
each output operation (for programs that mix C and C++ output conventions).
ios::unitbuf
ios::uppercase
Method: fmtflags ios::flags (fmtflags value)
Set value as the complete collection of flags controlling the format state. The flag values are described under `ios::flags ()'.
Use ios::setf or ios::unsetf to change one property at a
time.
Method: fmtflags ios::setf (fmtflags flag)
Set one particular flag (of those described for `ios::flags ()';
return the complete collection of flags previously in effect.
(Use ios::unsetf to cancel.)
Method: fmtflags ios::setf (fmtflags flag, fmtflags mask)
Clear the flag values indicated by mask, then set any of them that
are also in flag. (Flag values are described for `ios::flags
()'.) Return the complete collection of flags previously in
effect. (See ios::unsetf for another way of clearing flags.)
Method: fmtflags ios::unsetf (fmtflags flag)
Make certain flag (a combination of flag values described for
`ios::flags ()') is not set for this stream; converse of
ios::setf. Returns the old values of those flags.
For convenience, manipulators provide a way to change certain properties of streams, or otherwise affect them, in the middle of expressions involving `<<' or `>>'. For example, you might write
cout << "|" << setfill('*') << setw(5) << 234 << "|";
to produce `|**234|' as output.
Skip whitespace.
Flush an output stream. For example, `cout << ... <<flush;' has the same effect as `cout << ...; cout.flush();'.
Write an end of line character `\n', then flushes the output stream.
Write `\0' (the string terminator character).
Manipulator: setprecision (int signif)
You can change the value of ios::precision in `<<'
expressions with the manipulator `setprecision(signif)'; for
example,
cout << setprecision(2) << 4.567;
prints `4.6'. Requires `#include <iomanip.h>'.
You can change the value of ios::width in `<<' expressions
with the manipulator `setw(n)'; for example,
cout << setw(5) << 234;
prints ` 234' with two leading blanks. Requires `#include <iomanip.h>'.
Manipulator: setbase (int base)
Where base is one of 10 (decimal), 8 (octal), or
16 (hexadecimal), change the base value for numeric
representations. Requires `#include <iomanip.h>'.
Select decimal base; equivalent to `setbase(10)'.
Select hexadecimal base; equivalent to `setbase(16)'.
Select octal base; equivalent to `setbase(8)'.
Manipulator: setfill (char padding)
Set the padding character, in the same way as ios::fill.
Requires `#include <iomanip.h>'.
A related collection of methods allows you to extend this collection of flags and parameters for your own applications, without risk of conflict between them:
Method: static fmtflags ios::bitalloc ()
Reserve a bit (the single bit on in the result) to use as a flag. Using
bitalloc guards against conflict between two packages that use
ios objects for different purposes.
This method is available for upward compatibility, but is not in the
ANSI working paper. The number of bits available is limited; a
return value of 0 means no bit is available.
Method: static int ios::xalloc ()
Reserve space for a long integer or pointer parameter. The result is a
unique nonnegative integer. You can use it as an index to
ios::iword or ios::pword. Use xalloc to arrange
for arbitrary special-purpose data in your ios objects, without
risk of conflict between packages designed for different purposes.
Method: long& ios::iword (int index)
Return a reference to arbitrary data, of long integer type, stored in an
ios instance. index, conventionally returned from
ios::xalloc, identifies what particular data you need.
Method: long ios::iword (int index) const
Return the actual value of a long integer stored in an ios.
Method: void*& ios::pword (int index)
Return a reference to an arbitrary pointer, stored in an ios
instance. index, originally returned from ios::xalloc,
identifies what particular pointer you need.
Method: void* ios::pword (int index) const
Return the actual value of a pointer stored in an ios.
You can use these methods to synchronize related streams with one another:
Method: ostream* ios::tie () const
Report on what output stream, if any, is to be flushed before accessing
this one. A pointer value of 0 means no stream is tied.
Method: ostream* ios::tie (ostream* assoc)
Declare that output stream assoc must be flushed before accessing this stream.
Method: int ios::sync_with_stdio ([int switch])
Unless iostreams and C stdio are designed to work together, you
may have to choose between efficient C++ streams output and output
compatible with C stdio. Use `ios::sync_with_stdio()' to
select C compatibility.
The argument switch is a GNU extension; use 0 as the
argument to choose output that is not necessarily compatible with C
stdio. The default value for switch is 1.
If you install the stdio implementation that comes with GNU
libio, there are compatible input/output facilities for both C
and C++. In that situation, this method is unnecessary--but you may
still want to write programs that call it, for portability.
streambufFinally, you can use this method to access the underlying object:
Method: streambuf* ios::rdbuf () const
Return a pointer to the streambuf object that underlies this
ios.
ostream
Objects of class ostream inherit the generic methods from
ios, and in addition have the following methods available.
Declarations for this class come from `iostream.h'.
Constructor: ostream::ostream ()
The simplest form of the constructor for an ostream simply
allocates a new ios object.
Constructor: ostream::ostream (streambuf* sb [, ostream tie])
This alternative constructor requires a first argument sb of type
streambuf*, to use an existing open stream for output. It also
accepts an optional second argument tie, to specify a related
ostream* as the initial value for ios::tie.
If you give the ostream a streambuf explicitly, using
this constructor, the sb is not destroyed (or deleted or
closed) when the ostream is destroyed.
ostream
These methods write on an ostream (you may also use the operator
<<; see section Operators and Default Streams).
Method: ostream& ostream::put (char c)
Write the single character c.
Method: ostream& ostream::write (string, int length)
Write length characters of a string to this ostream,
beginning at the pointer string.
string may have any of these types: char*, unsigned
char*, signed char*.
Method: ostream& ostream::form (const char *format, ...)
A GNU extension, similar to fprintf(file,
format, ...).
format is a printf-style format control string, which is used
to format the (variable number of) arguments, printing the result on
this ostream. See ostream::vform for a version that uses
an argument list rather than a variable number of arguments.
Method: ostream& ostream::vform (const char *format, va_list args)
A GNU extension, similar to vfprintf(file,
format, args).
format is a printf-style format control string, which is used
to format the argument list args, printing the result on
this ostream. See ostream::form for a version that uses a
variable number of arguments rather than an argument list.
ostreamYou can control the output position (on output streams that actually support positions, typically files) with these methods:
Method: streampos ostream::tellp ()
Return the current write position in the stream.
Method: ostream& ostream::seekp (streampos loc)
Reset the output position to loc (which is usually the result of a
previous call to ostream::tellp). loc specifies an
absolute position in the output stream.
Method: ostream& ostream::seekp (streamoff loc, rel)
Reset the output position to loc, relative to the beginning, end,
or current output position in the stream, as indicated by rel (a
value from the enumeration ios::seekdir):
beg
cur
end
ostream utilities
You may need to use these ostream methods for housekeeping:
Deliver any pending buffered output for this ostream.
opfx is a prefix method for operations on ostream
objects; it is designed to be called before any further processing. See
ostream::osfx for the converse.
opfx tests that the stream is in state good, and if so
flushes any stream tied to this one.
The result is 1 when opfx succeeds; else (if the stream state is
not good), the result is 0.
osfx is a suffix method for operations on ostream
objects; it is designed to be called at the conclusion of any processing. All
the ostream methods end by calling osfx. See
ostream::opfx for the converse.
If the unitbuf flag is set for this stream, osfx flushes
any buffered output for it.
If the stdio flag is set for this stream, osfx flushes any
output buffered for the C output streams `stdout' and `stderr'.
istream
Class istream objects are specialized for input; as for
ostream, they are derived from ios, so you can use any of
the general-purpose methods from that base class. Declarations for this
class also come from `iostream.h'.
Constructor: istream::istream ()
When used without arguments, the istream constructor simply
allocates a new ios object and initializes the input counter (the
value reported by istream::gcount) to 0.
Constructor: istream::istream (streambuf *sb [, ostream tie])
You can also call the constructor with one or two arguments. The first
argument sb is a streambuf*; if you supply this pointer,
the constructor uses that streambuf for input.
You can use the second optional argument tie to specify a related
output stream as the initial value for ios::tie.
If you give the istream a streambuf explicitly, using
this constructor, the sb is not destroyed (or deleted or
closed) when the ostream is destroyed.
Use these methods to read a single character from the input stream:
Read a single character (or EOF) from the input stream, returning
it (coerced to an unsigned char) as the result.
Method: istream& istream::get (char& c)
Read a single character from the input stream, into &c.
Return the next available input character, but without changing the current input position.
Use these methods to read strings (for example, a line at a time) from the input stream:
Method: istream& istream::get (char* c, int len [, char delim])
Read a string from the input stream, into the array at c.
The remaining arguments limit how much to read: up to `len-1'
characters, or up to (but not including) the first occurrence in the
input of a particular delimiter character delim---newline
(\n) by default. (Naturally, if the stream reaches end of file
first, that too will terminate reading.)
If delim was present in the input, it remains available as if
unread; to discard it instead, see iostream::getline.
get writes `\0' at the end of the string, regardless
of which condition terminates the read.
Method: istream& istream::get (streambuf& sb [, char delim])
Read characters from the input stream and copy them on the
streambuf object sb. Copying ends either just before the
next instance of the delimiter character delim (newline \n
by default), or when either stream ends. If delim was present in
the input, it remains available as if unread.
Method: istream& istream::getline (charptr, int len [, char delim])
Read a line from the input stream, into the array at charptr.
charptr may be any of three kinds of pointer: char*,
unsigned char*, or signed char*.
The remaining arguments limit how much to read: up to (but not
including) the first occurrence in the input of a line delimiter
character delim---newline (\n) by default, or up to
`len-1' characters (or to end of file, if that happens sooner).
If getline succeeds in reading a "full line", it also discards
the trailing delimiter character from the input stream. (To preserve it
as available input, see the similar form of iostream::get.)
If delim was not found before len characters or end
of file, getline sets the ios::fail flag, as well as the
ios::eof flag if appropriate.
getline writes a null character at the end of the string, regardless
of which condition terminates the read.
Method: istream& istream::read (pointer, int len)
Read len bytes into the location at pointer, unless the input ends first.
pointer may be of type char*, void*, unsigned
char*, or signed char*.
If the istream ends before reading len bytes, read
sets the ios::fail flag.
Method: istream& istream::gets (char **s [, char delim])
A GNU extension, to read an arbitrarily long string
from the current input position to the next instance of the delim
character (newline \n by default).
To permit reading a string of arbitrary length, gets allocates
whatever memory is required. Notice that the first argument s is
an address to record a character pointer, rather than the pointer
itself.
Method: istream& istream::scan (const char *format ...)
A GNU extension, similar to fscanf(file,
format, ...). The format is a scanf-style format
control string, which is used to read the variables in the remainder of
the argument list from the istream.
Method: istream& istream::vscan (const char *format, va_list args)
Like istream::scan, but takes a single va_list argument.
istreamUse these methods to control the current input position:
Method: streampos istream::tellg ()
Return the current read position, so that you can save it and return to
it later with istream::seekg.
Method: istream& istream::seekg (streampos p)
Reset the input pointer (if the input device permits it) to p,
usually the result of an earlier call to istream::tellg.
Method: istream& istream::seekg (streamoff offset, ios::seek_dir ref)
Reset the input pointer (if the input device permits it) to offset characters from the beginning of the input, the current position, or the end of input. Specify how to interpret offset with one of these values for the second argument:
ios::beg
ios::cur
ios::end
istream utilities
Use these methods for housekeeping on istream objects:
Method: int istream::gcount ()
Report how many characters were read from this istream in the
last unformatted input operation.
Method: int istream::ipfx (int keepwhite)
Ensure that the istream object is ready for reading; check for
errors and end of file and flush any tied stream. ipfx skips
whitespace if you specify 0 as the keepwhite
argument, and ios::skipws is set for this stream.
To avoid skipping whitespace (regardless of the skipws setting on
the stream), use 1 as the argument.
Call istream::ipfx to simplify writing your own methods for reading
istream objects.
A placeholder for compliance with the draft ANSI standard; this method does nothing whatever.
If you wish to write portable standard-conforming code on istream
objects, call isfx after any operation that reads from an
istream; if istream::ipfx has any special effects that
must be cancelled when done, istream::isfx will cancel them.
Method: istream& istream::ignore ([int n] [, int delim])
Discard some number of characters pending input. The first optional
argument n specifies how many characters to skip. The second
optional argument delim specifies a "boundary" character:
ignore returns immediately if this character appears in the
input.
By default, delim is EOF; that is, if you do not specify a
second argument, only the count n restricts how much to ignore
(while input is still available).
If you do not specify how many characters to ignore, ignore
returns after discarding only one character.
Method: istream& istream::putback (char ch)
Attempts to back up one character, replacing the character backed-up
over by ch. Returns EOF if this is not allowed. Putting
back the most recently read character is always allowed. (This method
corresponds to the C function ungetc.)
Method: istream& istream::unget ()
Attempt to back up one character.
iostream
If you need to use the same stream for input and output, you can use an
object of the class iostream, which is derived from both
istream and ostream.
The constructors for iostream behave just like the constructors
for istream.
Constructor: iostream::iostream ()
When used without arguments, the iostream constructor simply
allocates a new ios object, and initializes the input counter
(the value reported by istream::gcount) to 0.
Constructor: iostream::iostream (streambuf* sb [, ostream* tie])
You can also call a constructor with one or two arguments. The first
argument sb is a streambuf*; if you supply this pointer,
the constructor uses that streambuf for input and output.
You can use the optional second argument tie (an ostream*)
to specify a related output stream as the initial value for
ios::tie.
As for ostream and istream, iostream simply uses
the ios destructor. However, an iostream is not deleted by
its destructor.
You can use all the istream, ostream, and ios
methods with an iostream object.
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