# std::vwprintf, std::vfwprintf, std::vswprintf

< cpp‎ | io‎ | c

C++
 Language Standard Library Headers Freestanding and hosted implementations Named requirements Language support library Concepts library (C++20) Diagnostics library Utilities library Strings library Containers library Iterators library Ranges library (C++20) Algorithms library Numerics library Input/output library Localizations library Regular expressions library (C++11) Atomic operations library (C++11) Thread support library (C++11) Filesystem library (C++17) Technical Specifications

Input/output library
 I/O manipulators C-style I/O Buffers basic_streambuf basic_filebuf basic_stringbuf strstreambuf(deprecated in C++98) basic_syncbuf(C++20) Streams Abstractions ios_base basic_ios basic_istream basic_ostream basic_iostream File I/O basic_ifstream basic_ofstream basic_fstream String I/O basic_istringstream basic_ostringstream basic_stringstream Array I/O istrstream(deprecated in C++98) ostrstream(deprecated in C++98) strstream(deprecated in C++98) Synchronized Output basic_osyncstream(C++20) Types streamoff streamsize fpos Error category interface iostream_category(C++11) io_errc(C++11)

C-style I/O
Functions
File access
Direct input/output
Unformatted input/output
 fgetc fgets fputc fputs getchar gets(until C++14) putchar puts ungetc
Formatted input/output
 scanffscanfsscanf vscanfvfscanfvsscanf(C++11)(C++11)(C++11) printffprintfsprintfsnprintf(C++11)
 wscanffwscanfswscanf vwscanfvfwscanfvswscanf(C++11)(C++11)(C++11) wprintffwprintfswprintf vwprintfvfwprintfvswprintf
File positioning
Error handling
Operations on files

 Defined in header  int vwprintf( const wchar_t* format, va_list vlist ); (1) int vfwprintf( std::FILE* stream, const wchar_t* format, va_list vlist ); (2) int vswprintf( const wchar_t* buffer, std::size_t buf_size,                const wchar_t* format, va_list vlist ); (3)

Loads the data from locations, defined by vlist,, converts them to wide string equivalents and writes the results to a variety of sinks.

1) Writes the results to stdout.
2) Writes the results to a file stream stream.
3) Writes the results to a wide string buffer. At most size-1 wide characters are written followed by null wide character.

### Parameters

stream - output wide stream to write to
buffer - pointer to a wide string to write to
buf_size - maximum number of wide characters to write
format - pointer to a null-terminated wide string specifying how to interpret the data.

The format string consists of ordinary wide characters (except %), which are copied unchanged into the output stream, and conversion specifications. Each conversion specification has the following format:

• introductory % character
• (optional) one or more flags that modify the behavior of the conversion:
• -: the result of the conversion is left-justified within the field (by default it is right-justified)
• +: the sign of signed conversions is always prepended to the result of the conversion (by default the result is preceded by minus only when it is negative)
• space: if the result of a signed conversion does not start with a sign character, or is empty, space is prepended to the result. It is ignored if + flag is present.
• # : alternative form of the conversion is performed. See the table below for exact effects otherwise the behavior is undefined.
• 0 : for integer and floating point number conversions, leading zeros are used to pad the field instead of space characters. For integer numbers it is ignored if the precision is explicitly specified. For other conversions using this flag results in undefined behavior. It is ignored if - flag is present.
• (optional) integer value or * that specifies minimum field width. The result is padded with space characters (by default), if required, on the left when right-justified, or on the right if left-justified. In the case when * is used, the width is specified by an additional argument of type int. If the value of the argument is negative, it results with the - flag specified and positive field width. (Note: This is the minimum width: The value is never truncated.)
• (optional) . followed by integer number or *, or neither that specifies precision of the conversion. In the case when * is used, the precision is specified by an additional argument of type int. If the value of this argument is negative, it is ignored. If neither a number nor * is used, the precision is taken as zero. See the table below for exact effects of precision.
• (optional) length modifier that specifies the size of the argument
• conversion format specifier

The following format specifiers are available:

Conversion
specifier
Explanation Argument type
length modifier hh

(C++11)

h (none) l ll

(C++11)

j

(C++11)

z

(C++11)

t

(C++11)

L
% writes literal %. The full conversion specification must be %%. N/A N/A N/A N/A N/A N/A N/A N/A N/A
c
writes a single character.

The argument is first converted to wchar_t as if by calling btowc. If the l modifier is used, the wint_t argument is first converted to wchar_t.

N/A N/A
int
wint_t
N/A N/A N/A N/A N/A
s
writes a character string

The argument must be a pointer to the initial element of a character array containing a multibyte character sequence beginning in the initial shift state, which is converted to wide character array as if by a call to mbrtowc with zero-initialized conversion state. Precision specifies the maximum number of wide characters to be written. If Precision is not specified, writes every wide characters up to and not including the first null terminator. If the l specifier is used, the argument must be a pointer to the initial element of an array of wchar_t.

N/A N/A
char*
wchar_t*
N/A N/A N/A N/A N/A
d
i
converts a signed integer into decimal representation [-]dddd.

Precision specifies the minimum number of digits to appear. The default precision is 1.
If both the converted value and the precision are 0 the conversion results in no characters.

signed char
short
int
long
long long
intmax_t
signed size_t
ptrdiff_t
N/A
o
converts a unsigned integer into octal representation oooo.

Precision specifies the minimum number of digits to appear. The default precision is 1. If both the converted value and the precision are 0 the conversion results in no characters. In the alternative implementation precision is increased if necessary, to write one leading zero. In that case if both the converted value and the precision are 0, single 0 is written.

unsigned char
unsigned short
unsigned int
unsigned long
unsigned long long
uintmax_t
size_t
unsigned version of ptrdiff_t
N/A
x
X
converts an unsigned integer into hexadecimal representation hhhh.

For the x conversion letters abcdef are used.
For the X conversion letters ABCDEF are used.
Precision specifies the minimum number of digits to appear. The default precision is 1. If both the converted value and the precision are 0 the conversion results in no characters. In the alternative implementation 0x or 0X is prefixed to results if the converted value is nonzero.

N/A
u
converts an unsigned integer into decimal representation dddd.

Precision specifies the minimum number of digits to appear. The default precision is 1. If both the converted value and the precision are 0 the conversion results in no characters.

N/A
f
F
converts floating-point number to the decimal notation in the style [-]ddd.ddd.

Precision specifies the minimum number of digits to appear after the decimal point character. The default precision is 6. In the alternative implementation decimal point character is written even if no digits follow it. For infinity and not-a-number conversion style see notes.

N/A N/A
double
double (C++11)
N/A N/A N/A N/A
long double
e
E
converts floating-point number to the decimal exponent notation.

For the e conversion style [-]d.ddde±dd is used.
For the E conversion style [-]d.dddE±dd is used.
The exponent contains at least two digits, more digits are used only if necessary. If the value is 0, the exponent is also 0. Precision specifies the minimum number of digits to appear after the decimal point character. The default precision is 6. In the alternative implementation decimal point character is written even if no digits follow it. For infinity and not-a-number conversion style see notes.

N/A N/A N/A N/A N/A N/A
a
A

(C++11)

converts floating-point number to the hexadecimal exponent notation.

For the a conversion style [-]0xh.hhhp±d is used.
For the A conversion style [-]0Xh.hhhP±d is used.
The first hexadecimal digit is not 0 if the argument is a normalized floating point value. If the value is 0, the exponent is also 0. Precision specifies the minimum number of digits to appear after the decimal point character. The default precision is sufficient for exact representation of the value. In the alternative implementation decimal point character is written even if no digits follow it. For infinity and not-a-number conversion style see notes.

N/A N/A N/A N/A N/A N/A
g
G
converts floating-point number to decimal or decimal exponent notation depending on the value and the precision.

For the g conversion style conversion with style e or f will be performed.
For the G conversion style conversion with style E or F will be performed.
Let P equal the precision if nonzero, 6 if the precision is not specified, or 1 if the precision is 0. Then, if a conversion with style E would have an exponent of X:

• if P > X ≥ −4, the conversion is with style f or F and precision P − 1 − X.
• otherwise, the conversion is with style e or E and precision P − 1.

Unless alternative representation is requested the trailing zeros are removed, also the decimal point character is removed if no fractional part is left. For infinity and not-a-number conversion style see notes.

N/A N/A N/A N/A N/A N/A
n
returns the number of characters written so far by this call to the function.

The result is written to the value pointed to by the argument. The specification may not contain any flag, field width, or precision.

signed char*
short*
int*
long*
long long*
intmax_t*
signed size_t*
ptrdiff_t*
N/A
p writes an implementation defined character sequence defining a pointer. N/A N/A void* N/A N/A N/A N/A N/A N/A

The floating point conversion functions convert infinity to inf or infinity. Which one is used is implementation defined.

Not-a-number is converted to nan or nan(char_sequence). Which one is used is implementation defined.

The conversions F, E, G, A output INF, INFINITY, NAN instead.

Even though %c expects int argument, it is safe to pass a char because of the integer promotion that takes place when a variadic function is called.

The correct conversion specifications for the fixed-width character types (int8_t, etc) are defined in the header <cinttypes> (although PRIdMAX, PRIuMAX, etc is synonymous with %jd, %ju, etc).

The memory-writing conversion specifier %n is a common target of security exploits where format strings depend on user input and is not supported by the bounds-checked printf_s family of functions.

There is a sequence point after the action of each conversion specifier; this permits storing multiple %n results in the same variable or, as an edge case, printing a string modified by an earlier %n within the same call.

If a conversion specification is invalid, the behavior is undefined.

vlist - variable argument list containing the data to print

### Return value

1,2) Number of wide characters written if successful or negative value if an error occurred.
3) Number of wide characters written (not counting the terminating null wide character) if successful or negative value if an encoding error occurred or if the number of characters to be generated was equal or greater than size.

### Notes

While narrow strings provide std::vsnprintf, which makes it possible to determine the required output buffer size, there is no equivalent for wide strings, and in order to determine the buffer size, the program may need to call std::vswprintf, check the result value, and reallocate a larger buffer, trying again until successful.