31 Input/output library [input.output]

31.5 Iostreams base classes [iostreams.base]

31.5.2 Class ios_base [ios.base]

31.5.2.1 General [ios.base.general]

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namespace std { class ios_base { public: class failure; // see below // [ios.fmtflags], fmtflags using fmtflags = T1; static constexpr fmtflags boolalpha = unspecified; static constexpr fmtflags dec = unspecified; static constexpr fmtflags fixed = unspecified; static constexpr fmtflags hex = unspecified; static constexpr fmtflags internal = unspecified; static constexpr fmtflags left = unspecified; static constexpr fmtflags oct = unspecified; static constexpr fmtflags right = unspecified; static constexpr fmtflags scientific = unspecified; static constexpr fmtflags showbase = unspecified; static constexpr fmtflags showpoint = unspecified; static constexpr fmtflags showpos = unspecified; static constexpr fmtflags skipws = unspecified; static constexpr fmtflags unitbuf = unspecified; static constexpr fmtflags uppercase = unspecified; static constexpr fmtflags adjustfield = see below; static constexpr fmtflags basefield = see below; static constexpr fmtflags floatfield = see below; // [ios.iostate], iostate using iostate = T2; static constexpr iostate badbit = unspecified; static constexpr iostate eofbit = unspecified; static constexpr iostate failbit = unspecified; static constexpr iostate goodbit = see below; // [ios.openmode], openmode using openmode = T3; static constexpr openmode app = unspecified; static constexpr openmode ate = unspecified; static constexpr openmode binary = unspecified; static constexpr openmode in = unspecified; static constexpr openmode noreplace = unspecified; static constexpr openmode out = unspecified; static constexpr openmode trunc = unspecified; // [ios.seekdir], seekdir using seekdir = T4; static constexpr seekdir beg = unspecified; static constexpr seekdir cur = unspecified; static constexpr seekdir end = unspecified; class Init; // [fmtflags.state], fmtflags state 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); // [ios.base.locales], locales locale imbue(const locale& loc); locale getloc() const; // [ios.base.storage], storage static int xalloc(); long& iword(int idx); void*& pword(int idx); // destructor virtual ~ios_base(); // [ios.base.callback], callbacks enum event { erase_event, imbue_event, copyfmt_event }; using event_callback = void (*)(event, ios_base&, int idx); void register_callback(event_callback fn, int idx); ios_base(const ios_base&) = delete; ios_base& operator=(const ios_base&) = delete; static bool sync_with_stdio(bool sync = true); protected: ios_base(); private: static int index; // exposition only long* iarray; // exposition only void** parray; // exposition only }; }
1
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ios_base defines several member types:
  • (1.1)
    a type failure, defined as either a class derived from system_error or a synonym for a class derived from system_error;
  • (1.2)
    a class Init;
  • (1.3)
    three bitmask types, fmtflags, iostate, and openmode;
  • (1.4)
    an enumerated type, seekdir.
2
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It maintains several kinds of data:
  • (2.1)
    state information that reflects the integrity of the stream buffer;
  • (2.2)
    control information that influences how to interpret (format) input sequences and how to generate (format) output sequences;
  • (2.3)
    additional information that is stored by the program for its private use.
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[Note 1: 
For the sake of exposition, the maintained data is presented here as:
  • (3.1)
    static int index, specifies the next available unique index for the integer or pointer arrays maintained for the private use of the program, initialized to an unspecified value;
  • (3.2)
    long* iarray, points to the first element of an arbitrary-length long array maintained for the private use of the program;
  • (3.3)
    void** parray, points to the first element of an arbitrary-length pointer array maintained for the private use of the program.
— end note]

31.5.2.2 Types [ios.types]

31.5.2.2.1 Class ios_base​::​failure [ios.failure]

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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
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An implementation is permitted to define ios_base​::​failure as a synonym for a class with equivalent functionality to class ios_base​::​failure shown in this subclause.
[Note 1: 
When ios_base​::​failure is a synonym for another type, that type is required to provide a nested type failure to emulate the injected-class-name.
— end note]
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
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When throwing ios_base​::​failure exceptions, implementations should provide values of ec that identify the specific reason for the failure.
[Note 2: 
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]
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explicit failure(const string& msg, const error_code& ec = io_errc::stream);
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Effects: Constructs the base class with msg and ec.
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explicit failure(const char* msg, const error_code& ec = io_errc::stream);
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Effects: Constructs the base class with msg and ec.

31.5.2.2.2 Type ios_base​::​fmtflags [ios.fmtflags]

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using fmtflags = T1;
1
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The type fmtflags is a bitmask type ([bitmask.types]).
Setting its elements has the effects indicated in Table 118.
Table 118: fmtflags effects [tab:ios.fmtflags]
Element
Effect(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
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Type fmtflags also defines the constants indicated in Table 119.
Table 119: fmtflags constants [tab:ios.fmtflags.const]
Constant
Allowable values
adjustfield
left | right | internal
basefield
dec | oct | hex
floatfield
scientific | fixed

31.5.2.2.3 Type ios_base​::​iostate [ios.iostate]

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using iostate = T2;
1
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The type iostate is a bitmask type ([bitmask.types]) that contains the elements indicated in Table 120.
Table 120: iostate effects [tab:ios.iostate]
Element
Effect(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
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Type iostate also defines the constant:

31.5.2.2.4 Type ios_base​::​openmode [ios.openmode]

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using openmode = T3;
1
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The type openmode is a bitmask type ([bitmask.types]).
It contains the elements indicated in Table 121.
Table 121: openmode effects [tab:ios.openmode]
Element
Effect(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
noreplace
open in exclusive mode
out
open for output
trunc
truncate an existing stream when opening

31.5.2.2.5 Type ios_base​::​seekdir [ios.seekdir]

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using seekdir = T4;
1
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The type seekdir is an enumerated type ([enumerated.types]) that contains the elements indicated in Table 122.
Table 122: seekdir effects [tab:ios.seekdir]
Element
Meaning
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

31.5.2.2.6 Class ios_base​::​Init [ios.init]

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namespace std { class ios_base::Init { public: Init(); Init(const Init&) = default; ~Init(); Init& operator=(const Init&) = default; private: static int init_cnt; // exposition only }; }
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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>.
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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.
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Init();
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Effects: Constructs and initializes the objects cin, cout, cerr, clog, wcin, wcout, wcerr, and wclog if they have not already been constructed and initialized.
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~Init();
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Effects: 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().

31.5.2.3 State functions [fmtflags.state]

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fmtflags flags() const;
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Returns: The format control information for both input and output.
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fmtflags flags(fmtflags fmtfl);
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Postconditions: fmtfl == flags().
3
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Returns: The previous value of flags().
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fmtflags setf(fmtflags fmtfl);
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Effects: Sets fmtfl in flags().
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Returns: The previous value of flags().
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fmtflags setf(fmtflags fmtfl, fmtflags mask);
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Effects: Clears mask in flags(), sets fmtfl & mask in flags().
7
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Returns: The previous value of flags().
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void unsetf(fmtflags mask);
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Effects: Clears mask in flags().
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streamsize precision() const;
9
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Returns: The precision to generate on certain output conversions.
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streamsize precision(streamsize prec);
10
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Postconditions: prec == precision().
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Returns: The previous value of precision().
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streamsize width() const;
12
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Returns: The minimum field width (number of characters) to generate on certain output conversions.
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streamsize width(streamsize wide);
13
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Postconditions: wide == width().
14
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Returns: The previous value of width().

31.5.2.4 Functions [ios.base.locales]

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locale imbue(const locale& loc);
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Effects: Calls each registered callback pair (fn, idx) ([ios.base.callback]) as (*fn)(imbue_event, *this, idx) at such a time that a call to ios_base​::​getloc() from within fn returns the new locale value loc.
2
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Postconditions: loc == getloc().
3
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Returns: The previous value of getloc().
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locale getloc() const;
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Returns: If no locale has been imbued, a copy of the global C++ locale, locale(), in effect at the time of construction.
Otherwise, returns the imbued locale, to be used to perform locale-dependent input and output operations.

31.5.2.5 Static members [ios.members.static]

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static bool sync_with_stdio(bool sync = true);
1
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Effects: If any input or output operation has occurred using the standard streams prior to the call, the effect is implementation-defined.
Otherwise, called with a false argument, it allows the standard streams to operate independently of the standard C streams.
2
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Returns: true if the previous state of the standard iostream objects was synchronized and otherwise returns false.
The first time it is called, the function returns true.
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Remarks: When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by fputc(f, c); is the same as the effect of str.rdbuf()->sputc(c); for any sequences of characters; the effect of extracting a character c by c = fgetc(f); is the same as the effect of c = str.rdbuf()->sbumpc(); for any sequences of characters; and the effect of pushing back a character c by ungetc(c, f); is the same as the effect of str.rdbuf()->sputbackc(c); for any sequence of characters.269
269)269)
This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream.
In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer.

31.5.2.6 Storage functions [ios.base.storage]

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static int xalloc();
1
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Returns: index ++.
2
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Remarks: Concurrent access to this function by multiple threads does not result in a data race.
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long& iword(int idx);
3
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Preconditions: idx is a value obtained by a call to xalloc.
4
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Effects: If iarray is a null pointer, allocates an array of long of unspecified size and stores a pointer to its first element in iarray.
The function then extends the array pointed at by iarray as necessary to include the element iarray[idx].
Each newly allocated element of the array is initialized to zero.
The reference returned is invalid after any other operation on the object.270
However, the value of the storage referred to is retained, so that until the next call to copyfmt, calling iword with the same index yields another reference to the same value.
If the function fails271 and *this is a base class subobject of a basic_ios<> object or subobject, the effect is equivalent to calling basic_ios<>​::​setstate(badbit) on the derived object (which may throw failure).
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Returns: On success iarray[idx].
On failure, a valid long& initialized to 0.
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void*& pword(int idx);
6
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Preconditions: idx is a value obtained by a call to xalloc.
7
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Effects: If parray is a null pointer, allocates an array of pointers to void of unspecified size and stores a pointer to its first element in parray.
The function then extends the array pointed at by parray as necessary to include the element parray[idx].
Each newly allocated element of the array is initialized to a null pointer.
The reference returned is invalid after any other operation on the object.
However, the value of the storage referred to is retained, so that until the next call to copyfmt, calling pword with the same index yields another reference to the same value.
If the function fails272 and *this is a base class subobject of a basic_ios<> object or subobject, the effect is equivalent to calling basic_ios<>​::​setstate(badbit) on the derived object (which may throw failure).
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Returns: On success parray[idx].
On failure a valid void*& initialized to 0.
9
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Remarks: After a subsequent call to pword(int) for the same object, the earlier return value may no longer be valid.
270)270)
An implementation is free to implement both the integer array pointed at by iarray and the pointer array pointed at by parray as sparse data structures, possibly with a one-element cache for each.
271)271)
For example, because it cannot allocate space.
272)272)
For example, because it cannot allocate space.

31.5.2.7 Callbacks [ios.base.callback]

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void register_callback(event_callback fn, int idx);
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Preconditions: The function fn does not throw exceptions.
2
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Effects: Registers the pair (fn, idx) such that during calls to imbue() ([ios.base.locales]), copyfmt(), or ~ios_base() ([ios.base.cons]), the function fn is called with argument idx.
Functions registered are called when an event occurs, in opposite order of registration.
Functions registered while a callback function is active are not called until the next event.
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Remarks: Identical pairs are not merged.
A function registered twice will be called twice.

31.5.2.8 Constructors and destructor [ios.base.cons]

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ios_base();
1
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Effects: Each ios_base member has an indeterminate value after construction.
The object's members shall be initialized by calling basic_ios​::​init before the object's first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.
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~ios_base();
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Effects: Calls each registered callback pair (fn, idx) ([ios.base.callback]) as (*fn)(​erase_event, *this, idx) at such time that any ios_base member function called from within fn has well-defined results.
Then, any memory obtained is deallocated.