13.27 ios

13.27 <ios>

The <ios> header declares the classes, types, and manipulator functions that form the foundation of the C++ I/O library (which is often called I/O streams ). The class ios_base is the base class for all I/O stream classes. The class template basic_ios derives from ios_base and declares the behavior that is common to all I/O streams (e.g., establishing a stream buffer and defining the I/O state).

Refer to Chapter 9 for more information about input and output, including the use of manipulators, formatting flags, streams, and stream buffers.

The <ios> header #include s <iosfwd> .

basic_ios class template

Base class template for all I/O streams

 template <class charT, class traits = char_traits<charT> > class  basic_ios  : public ios_base { public:   typedef charT  char_type  ;   typedef typename traits::int_type  int_type  ;   typedef typename traits::pos_type  pos_type  ;   typedef typename traits::off_type  off_type  ;   typedef traits  traits_type  ;   // Status  operator void*  (  );   const bool  operator!  (  );   const iostate  rdstate  (  ) const;   void  clear  (iostate state = goodbit);   void  clear  (io_state state);   void  setstate  (iostate state);   void  setstate  (io_state state);   bool  good  (  ) const;   bool  eof  (  ) const;   bool  fail  (  ) const;   bool  bad  (  ) const;   iostate  exceptions  (  ) const;   void  exceptions  (iostate except);   void  exceptions  (io_state except);   explicit  basic_ios  (basic_streambuf<charT,traits>* sb);   virtual  ~basic_ios  (  );   basic_ostream<charT,traits>*  tie  (  ) const;   basic_ostream<charT,traits>*  tie  (basic_ostream<charT,traits>* tiestr);   basic_streambuf<charT,traits>*  rdbuf  (  ) const;   basic_streambuf<charT,traits>*  rdbuf  (basic_streambuf<charT,traits>* sb);   basic_ios&  copyfmt  (const basic_ios& rhs);   char_type  fill  (  ) const;   char_type  fill  (char_type ch);   locale  imbue  (const locale& loc);   char  narrow  (char_type c, char deflt) const;   char_type  widen  (char c) const; protected:  basic_ios  (  );   void  init  (basic_streambuf<charT,traits>* buf); private:  basic_ios  (const basic_ios& );           // Not defined   basic_ios&  operator=  (const basic_ios&); // Not defined };

The basic_ios class template is the root of all I/O stream class templates. It provides a common functionality for all derived stream classes; in particular, it manages a stream buffer. In the following descriptions, the name buf refers to a private data member that points to the stream buffer. An implementation can use any name.

The following are the member functions of basic_ios :

basic_ios ( )

The default constructor leaves the data members uninitialized . In a derived class, the default constructor must call init to initialize the members.

basic_ios (const basic_ios&)

The copy constructor is declared private and is not defined, which prevents the copying of any I/O stream objects.

explicit basic_ios (basic_streambuf<charT,traits>* sb)

Calls init(sb) to initialize the members.

operator void* ( )

If fail( ) returns true , the void* operator returns a null pointer; otherwise , it returns a non-null pointer to indicate success. operator void* is most often used as an implicit conversion in a conditional (e.g., while (cin) cin >> data[i++] ).

const bool operator! ( )

Returns fail( ) . operator ! is most often used in a conditional (e.g., if (!cout) cerr << " output error\n ").

basic_ios& operator= (const basic_ios&)

The assignment operator, like the copy constructor, is private, so it cannot be used and is not defined. Assigning or copying an I/O stream would corrupt the stream buffer.

bool bad ( ) const

Returns true if badbit is set in rdstate( ) , or false otherwise.

void clear (iostate state = goodbit)

void clear (io_state state)

Sets the I/O state to state . If rdbuf( ) is a null pointer, badbit is also set (to state ios_base::badbit ). After setting the state, if any state bit is an exception bit ( (rdstate( ) & exceptions( )) != ), basic_ios::failure is thrown.

The second form is deprecated. See ios_base::iostate later in this section for details.

basic_ios& copyfmt (const basic_ios& rhs)

Copies formatting information from rhs . In particular, the format flags, fill character, locale, and the contents of the iword( ) and pword( ) arrays are copied. The I/O state and stream buffer are not copied . Before copying any callback functions, each one is called with erase_event . The callbacks are then replaced with those copied from rhs , and each one is called with copyfmt_event . (See ios_base for information about callbacks.) The exceptions( ) mask is copied last. The return value is *this .

bool eof ( ) const

Returns true if eofbit is set in rdstate( ) , or false otherwise.

iostate exceptions ( ) const

void exceptions (iostate except)

void exceptions (io_state except)

Returns or sets the exception mask. (See the clear function for how and when an exception is thrown.) The third form is deprecated. See ios_base::iostate later in this section for details.

bool fail ( ) const

Returns true if badbit is set or if failbit is set in rdstate( ) , or false if neither bit is set.

char_type fill ( ) const

char_type fill (char_type ch)

Returns or changes the fill character (also called the pad character ). When setting the fill character, the old fill character is returned.

bool good ( ) const

Returns true if the I/O state is cleanthat is, it returns rdstate( ) == .

locale imbue (const locale& loc)

Calls ios_base::imbue(loc) and rdbuf( )->pubimbue(loc) (if rdbuf( ) is not null). The return value is the previous value of ios_base::imbue( ) .

void init (basic_streambuf<charT,traits>* buf)

Initializes the basic_ios object. Table 13-12 lists the observable effects of initialization. Also, the arrays for iword( ) and pword( ) are initially null pointers.

Table 13-12. Effects of calling basic_ios::init

Member function	Return value
`exceptions( )`	`goodbit`
`fill( )`	`widen(' ')`
`flags( )`	`skipws dec`
`getloc( )`	Current global locale, that is, `std::locale( )`
`precision( )`	`6`
`rdbuf( )`	`buf`
`rdstate( )`	`buf` `!=0 ? goodbit : badbit`
`tie( )`	Null pointer
`width( )`

char narrow (char_type c, char deflt) const

Narrows the character c by returning the following:

 std::use_facet<ctype<char_type> >(getloc(  )).narrow(c, deflt)

basic_streambuf<charT,traits>* rdbuf ( ) const

basic_streambuf<charT,traits>*

rdbuf (basic_streambuf<charT,traits>* sb)

Returns or changes the stream buffer, buf . After changing the stream buffer, the rdbuf function calls clear( ) . The function returns the previous value of rdbuf( ) .

const iostate rdstate ( ) const

Returns the current I/O state bitmask. See the bad , eof , fail , and good functions for convenient ways to test different bits in the state mask.

void setstate (iostate state)

void setstate (io_state state)

Sets the specified bits in the I/O state bitmaskthat is, it calls clear(rdstate( ) state) . The second form is deprecated. See ios_base::iostate later in this section for details.

basic_ostream<charT,traits>* tie ( ) const

basic_ostream<charT,traits>* tie (basic_ostream<charT,traits>* tiestr)

Ties a stream (typically an input stream) to an output stream, tiestr . Any input operation on this stream is prefaced by flushing tiestr . Tying streams can be used to ensure that prompts appear at the proper time. With no arguments, the tie function returns the currently tied stream, or if no stream is tied.

char_type widen (char c) const

Widens the character c by returning the following:

 std::use_facet<ctype<char_type> >(getloc(  )).widen(c)

The Shift State Problem

The C++ standard requires a library implementation to implement fpos<> and the streamoff types so that you can convert an object whose type is any fpos<> instance into a streamoff object and back again, recovering the original value.

Most library implementations do not work this way.

Instead, streamoff is usually implemented as an integral type, such as int or long . The problem is that fpos<> must store a stream position and a shift state, and the stream position is usually implemented with the same type as streamoff . In other words, converting from fpos<> to streamoff discards the shift state. The library cannot convert streamoff back to the original fpos<> because the original shift state is gone.

Note that char_traits has the same problem when specialized for char and wchar_t because its pos_type is defined to be streampos , which is defined in <iosfwd> to be fpos<mbstate_t> , and off_type is defined as streamoff .

The solution is not to convert a stream position to a stream offset. If you must do this, remember that you might be sacrificing the shift state. Save the shift state separately so you can restore it when converting a stream offset back to a stream position.

 class  ios_base  { public:   class  failure  ;   typedef  . . .  fmtflags  ;   typedef  . . .  iostate  ;   typedef  . . .  io_state  ;   typedef  . . .  openmode  ;   typedef  . . .  open_mode  ;   typedef  . . .  seekdir  ;   typedef  . . .  seek_dir  ;   typedef  . . .  streamoff  ;   typedef  . . .  streampos  ;   class  Init  ;   // Destructor   virtual  ~ios_base  (  );   // Formatting   fmtflags  flags  (  ) const;   fmtflags  flags  (fmtflags fmtfl);   fmtflags  setf  (fmtflags fmtfl);   fmtflags  setf  (fmtflags fmtfl, fmtflags mask);   void  unsetf  (fmtflags mask);   streamsize  precision  (  ) const;   streamsize  precision  (streamsize prec);   streamsize  width  (  ) const;   streamsize  width  (streamsize wide);   // Locales   locale  imbue  (const locale& loc);   locale  getloc  (  ) const;   // Storage   static int  xalloc  (  );   long&  iword  (int index);   void*&  pword  (int index);   // Callbacks   enum  event  { erase_event, imbue_event, copyfmt_event };   typedef void (*  event_callback  )(event, ios_base&, int index);   void  register_callback  (event_callback fn, int index);   static bool  sync_with_stdio  (bool sync = true); protected:  ios_base  (  ); private:  ios_base  (const ios_base&);   ios_base&  operator=  (const ios_base&); };

The ios_base class is the root class for all the I/O stream classes. It declares fundamental types that are used throughout the I/O library. It also has members to keep track of formatting for input and output, storing arbitrary information for derived classes, and registering functions to be called when something interesting happens to the stream object.

The io_state , open_mode , seek_dir , streamoff , and streampos types are deprecated and might not be included in a future revision of the C++ standard. The first three are integer types that are equivalent to iostate , openmode , and seekdir . (See their respective subsections later in this section for details.) The streamoff and streampos types have equivalent types at namespace scope. See streamoff later in this section and streampos in <iosfwd> for details.

The following are the member functions of ios_base :

ios_base ( )

The default constructor is protected so you cannot accidentally declare an object of type ios_base . It does not initialize its members. That is left to the basic_ios::init function.

ios_base (const ios_base&)

The copy constructor is private and not defined so you cannot copy objects of type ios_base or its derived classes.

virtual ~ios_base ( )

Calls every registered callback with the erase_event if the ios_base object has been properly initialized . See basic_ios::init .

ios_base& operator= (const ios_base&)

The assignment operator is private and not defined to prevent the assignment of ios_base objects or its derivatives.

fmtflags flags ( ) const

fmtflags flags (fmtflags fmtfl)

Returns the current format flags or sets the flags. When setting the flags, the previous flags are returned.

locale getloc ( ) const

Returns the stream's currently imbued locale.

locale imbue (const locale& loc)

Saves loc as the new locale and calls all registered callbacks with imbue_event . The new locale is stored before calling any callbacks, so if a callback function calls getloc , it gets the new locale.

long& iword (int index)

Returns a reference to a long integer that is stored in a private array, at index index . If iword has been called before with the same index, a reference to the array element is returned. Otherwise, the array is extended as needed so that index is a valid index, and the new entry is initialized to . A reference to the new element is returned.

The structure of the internal array is implementation-defined, and it might be a sparse array. The internal array grows as needed, and all prior values stored in the array are preserved, although the references might become invalid in any of the following situations:

After a call to iword with a different index
After calling basic_ios::copyfmt for this object
When the object is destroyed

If iword fails (perhaps because the internal array cannot grow), it returns a reference to a valid long& with a value that is initially . If the member function is called for an object whose class derives from basic_ios<> , badbit is set (which might throw ios_base::failure ).

See the xalloc member function to learn how to obtain a suitable index.

streamsize precision ( ) const

streamsize precision (streamsize prec)

Returns or sets the precision (places after the decimal point) used to format floating-point numbers for output. When setting a new precision, the previous precision is returned.

void*& pword (int index)

Returns a reference to a void* that is stored in a private array, at index index . If pword has been called before with the same index, a reference to the array element is returned. Otherwise, the array is extended as needed so that index is a valid index, and the new entry is initialized to a null pointer. A reference to the new element is returned.

After a call to pword with a different index
After calling basic_ios::copyfmt for this object
When the object is destroyed

If pword fails (perhaps because the internal array cannot grow), it returns a reference to a valid void*& with a value that is initially . If the object derives from basic_ios<> , badbit is set (which might throw ios_base::failure ).

See the xalloc member function to learn how to obtain a suitable index.

void register_callback (event_callback fn, int index)

Registers a function fn to be called when one of three events occurs for the ios_base object:

The object is destroyed ( erase_event )
copyfmt is called ( erase_event followed by copyfmt_event )
imbue is called ( imbue_event )

Each callback function is registered with an integer index . The index is passed to the callback function. Functions are called in the opposite order of registration. The callback function must not throw exceptions.

For example, suppose a program stores some debugging information with each stream. It allocates a struct and stores a pointer to the struct in the stream's pword array. When copyfmt is called, the debugging information should also be copied. Example 13-14 shows how to use callbacks to make sure the memory is managed properly.

Example 13-14. Copying information associated with streams

 void manage_info(std::ios_base::event event,                  std::ios_base& stream, int index) {   infostruct* ip;       switch(event) {   case std::ios_base::erase_event:     ip = static_cast<infostruct*>(stream.pword(index));     stream.pword(index) = 0;     delete ip;     break;   case std::ios_base::copyfmt_event:     stream.pword(index) = new infostruct;     break;   default:     break; // imbue_event does not affect storage.   } }     void openread(std::ifstream& f, const char* name) {   f.open(name);   int index = f.xalloc(  );   f.pword(index) = new infostruct;   f.register_callback(manage_info, index); }

fmtflags setf (fmtflags addflags)

Sets the addflags bits of the formatting flags. It is equivalent to calling flags(flags( ) addflags) .

fmtflags setf (fmtflags newflags, fmtflags mask)

Clears the mask bits from the formatting flags and then sets the newflags & mask bits. It is equivalent to calling flags((flags( ) & ~mask) (newflags & mask)) . The two-argument version of setf is most often used with multiple-choice flags (e.g., setf(ios_base::dec , ios_base::basefield) ).

static bool sync_with_stdio (bool sync = true)

Determines whether the standard C++ I/O objects are synchronized with the C I/O functions. Initially, they are synchronized.

If you call sync_with_stdio(false) after any I/O has been performed, the behavior is implementation-defined.

void unsetf (fmtflags mask)

Clears the mask bits from the formatting flags. It is equivalent to calling flags(flags( ) & ~mask) .

streamsize width ( ) const

streamsize width (streamsize wide)

Returns or sets the minimum field width. When setting the width, the previous width is returned.

static int xalloc ( )

Returns a unique integer, suitable for use as an index to the iword or pword functions. You can think of ios_base as having a static integer data member, xalloc_index , and xalloc is implemented so it returns xalloc_index ++ .

Literal name	Description
`boolalpha`	Reads and writes bool values as text, according to the locale
`dec`	Reads and writes decimal integers
`fixed`	Writes floating-point values in fixed notation
`hex`	Reads and writes hexadecimal integers
`internal`	Aligns output to internal point (e.g., after sign or `0x` )
`left`	Left-aligns output
`oct`	Reads and writes octal integers
`right`	Right-aligns output
`scientific`	Writes floating-point values in scientific notation
`showbase`	Writes a prefix for an integer radix ( e.g., `0x` for hexadecimal)
`showpoint`	Writes decimal point, even if not needed
`showpos`	Writes plus sign ( `+` ), even if not needed
`skipws`	Skips whitespace before input
`unitbuf`	Flushes output after each operation
`uppercase`	Uses uppercase in generated output (e.g., `0X` prefix)

Constant name	Value	Default
`adjustfield`	`left` `internal` `right`	`right`
`basefield`	`dec` `hex` `oct`	Output: `dec` Input: leading `0x` or `0x` is `hex` , is `oct` , anything else is `dec`
`floatfield`	`fixed` `scientific`	`scientific` if exponent is < -4 or precision, else `fixed` ; strip trailing zeros and unneeded decimal point

Example

Example 13-15. Ensuring proper initialization of standard I/O streams

 class myclass { public:   myclass(  ) {     if (! okay(  ))       std::cerr << "Oops: not okay!\n";   } }; static std::ios_base::Init init; static myclass myobject;

Literal	Description
`app`	Seeks to end-of-file before each write
`ate`	Seeks to end-of-file immediately after opening
`binary`	Reads and writes in binary mode (default is text)
`in`	Opens for input (reading)
`out`	Opens for output (writing)
`trunc`	Truncates file to zero length

Literal	Description
`beg`	Seeks from the beginning of the stream (e.g., offset is absolute position)
`cur`	Seeks from the current position; positive is toward end of stream, and negative offsets are toward the beginning of the stream
`end`	Seeks relative to the end of the stream; negative offsets are towards the beginning

Literal	Description
`badbit`	Irrecoverable error, such as a null `streambuf` pointer or a write failure
`eofbit`	End-of-file when reading
`failbit`	Failure to read or write expected characters ( e.g., trying to read an integer from nonnumeric input)
`goodbit`	No problems; value is

13.27 <ios>

Table 13-12. Effects of calling basic_ios::init

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The Shift State Problem

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Example 13-14. Copying information associated with streams

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Table 13-13. fmtflags literals

Table 13-14. fmtflags constants

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Example

Example 13-15. Ensuring proper initialization of standard I/O streams

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Table 13-15. iostate literals

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Table 13-16. openmode literals

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Table 13-17. seekdir literals

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