std::wprint...d::swprintf(3)

πŸ“¦ version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

✨ std::wprint...d::swprintf(3) β€” C++ Standard Libary ✨

okay so this is the std::wprint...d::swprintf(3) page from the C++ Standard Libary, and yes the manual name is literally truncated like that. We are not fixing it, we are respecting the vibe. The actual main characters are std::wprintf, std::fwprintf, and std::swprintf. πŸ’…

πŸ’– NAME πŸ’–

std::wprintf, std::fwprintf, std::swprintf β€” that’s the whole name section, bestie. It’s the std::wprintf, std::fwprintf, std::swprintf trio, no cap.

🌈 SYNOPSIS 🌈

Defined in header <cwchar>.

int wprintf( const wchar_t* format, ... );       (1)
int fwprintf( std::FILE* stream, const wchar_t* format, ... );       (2)
int swprintf( wchar_t* buffer, std::size_t size, const wchar_t* format, ... ); (3)

Loads the data from the given locations, 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 β€” output file stream to write to.
  • buffer β€” pointer to a wide character string to write to.
  • size β€” up to size - 1 characters may be written, plus the null terminator.
  • format β€” pointer to a null-terminated wide string specifying how to interpret the data arguments specifying data to print.
  • ... β€” arguments specifying data to print.
big warning, bestie: if any argument after default conversions is not the type expected by the corresponding conversion specifier, or if there are fewer arguments than required by format, the behavior is undefined. If there are more arguments than required by format, the extraneous arguments are evaluated and ignored. 😭

πŸ’… the format string, explained πŸ’…

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, which appears before the argument to be converted and the argument supplying precision if one is supplied. 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, which appears before the argument to be converted, but after the argument supplying minimum field width if one is supplied. 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 (in combination with the conversion format specifier, it specifies the type of the corresponding argument).
  • conversion format specifier.

🌟 the following format specifiers are available 🌟

The length modifier columns are hh, h, (none), l, ll, j, z, t, and L. In the original table, hh, ll, j, z, and t are marked (C++11).

  • % β€” Writes literal %. The full conversion specification must be %%. Expected argument type: N/A for every length modifier (hh, h, (none), l, ll, j, z, t, L).
  • c β€” 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. Expected argument type: hh β†’ N/A; h β†’ N/A; (none) β†’ int; l β†’ wint_t; ll β†’ N/A; j β†’ N/A; z β†’ N/A; t β†’ N/A; L β†’ N/A.
  • s β€” 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 state. Precision specifies the maximum number of wide characters to be written. If Precision is not specified, writes every wide character 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. Expected argument type: hh β†’ N/A; h β†’ N/A; (none) β†’ char*; l β†’ wchar_t*; ll β†’ N/A; j β†’ N/A; z β†’ N/A; t β†’ N/A; L β†’ N/A.
  • d, i β€” 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. Expected argument type: hh β†’ signed char; h β†’ short; (none) β†’ int; l β†’ long; ll β†’ long long; j β†’ intmax_t; z β†’ signed size_t; t β†’ ptrdiff_t; L β†’ N/A.
  • o β€” Converts an 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. Expected argument type: hh β†’ unsigned char; h β†’ unsigned short; (none) β†’ unsigned int; l β†’ unsigned long; ll β†’ unsigned long long; j β†’ uintmax_t; z β†’ size_t; t β†’ unsigned ptrdiff_t; L β†’ N/A.
  • x, X β€” Converts an unsigned integer into hexadecimal representation hhhh. For the x conversion letters abcdef are used. For the X version of the 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. Expected argument type: hh β†’ unsigned char; h β†’ unsigned short; (none) β†’ unsigned int; l β†’ unsigned long; ll β†’ unsigned long long; j β†’ uintmax_t; z β†’ size_t; t β†’ unsigned ptrdiff_t; L β†’ N/A.
  • u β€” 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. Expected argument type: hh β†’ unsigned char; h β†’ unsigned short; (none) β†’ unsigned int; l β†’ unsigned long; ll β†’ unsigned long long; j β†’ uintmax_t; z β†’ size_t; t β†’ unsigned ptrdiff_t; L β†’ N/A.
  • the floating-point row (cut off right here in part 1) β€” Converts floating-point number to the decimal notation in the style [-]ddd.ddd. The original page literally stops mid-table at this exact point, so the rest of this row is off living its best life in part 2. πŸ’€βœ¨
  • f, F β€” bestie, this is the precision one πŸ’… it specifies the exact number of digits to appear after the decimal point character. The default precision is 6. In the alternative implementation the decimal point character is written even if no digits follow it (yes, even then πŸŽ€). For infinity and not-a-number conversion style, see the notes below, iykyk.
    • πŸ’¬ argument types by length modifier: hh β†’ N/A, h β†’ N/A, no modifier β†’ double, l β†’ double (C++11), ll β†’ N/A, j β†’ N/A, z β†’ N/A, t β†’ N/A, L β†’ long double.
  • e, E β€” this one converts a floating-point number to the decimal exponent notation ✨
    • For the e conversion style, [-]d.dddeΒ±dd is used.
    • For the E conversion style, [-]d.dddEΒ±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 exact number of digits to appear after the decimal point character. The default precision is 6.
    • In the alternative implementation the decimal point character is written even if no digits follow it.
    • For infinity and not-a-number conversion style see notes. 🌸
    • πŸ’¬ argument types by length modifier: hh β†’ N/A, h β†’ N/A, no modifier β†’ double, l β†’ double (C++11), ll β†’ N/A, j β†’ N/A, z β†’ N/A, t β†’ N/A, L β†’ long double.
  • a, A β€” hex exponent notation, highkey iconic πŸ”₯
    • For the a conversion style, [-]0xh.hhhpΒ±d is used.
    • For the A conversion style, [-]0Xh.hhhPΒ±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 exact number of digits to appear after the hexadecimal point character. The default precision is sufficient for exact representation of the value.
    • In the alternative implementation the decimal point character is written even if no digits follow it.
    • For infinity and not-a-number conversion style see notes.
    • πŸ’¬ argument types by length modifier: hh β†’ N/A, h β†’ N/A, no modifier β†’ double, l β†’ double (C++11), ll β†’ N/A, j β†’ N/A, z β†’ N/A, t β†’ N/A, L β†’ long double.
  • g, G β€” the vibe-check conversion πŸ¦‹ it picks decimal or decimal exponent notation depending on the value and the precision.
    • For the g conversion style, a conversion with style e or f will be performed.
    • For the G conversion style, a 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 β‰₯ βˆ’4, the conversion is with style f or F and precision P βˆ’ 1 βˆ’ X.
      • otherwise, the conversion is with style e or E and precision P βˆ’ 1.
    • Unless alternative representation is requested, the trailing zeros are removed β€” and the decimal point character is removed too if no fractional part is left. For infinity and not-a-number conversion style see notes. ✨
    • πŸ’¬ argument types by length modifier: hh β†’ N/A, h β†’ N/A, no modifier β†’ double, l β†’ double (C++11), ll β†’ N/A, j β†’ N/A, z β†’ N/A, t β†’ N/A, L β†’ long double.
  • n β€” returns the number of characters written so far by this call to the function, and the result is written into the value pointed to by the argument 🫢
    • πŸ’¬ argument types by length modifier: hh β†’ signed char*, h β†’ signed short*, no modifier β†’ int*, l β†’ long*, ll β†’ long long*, j β†’ intmax_t*, z β†’ size_t*, t β†’ ptrdiff_t*, L β†’ N/A.
    • 🚨 the specification may not contain any flag, field width, or precision. Periodt.
  • p β€” writes an implementation-defined character sequence defining a pointer πŸ‘‘
    • πŸ’¬ argument types by length modifier: hh β†’ N/A, h β†’ N/A, no modifier β†’ void*, l β†’ N/A, ll β†’ N/A, j β†’ N/A, z β†’ N/A, t β†’ N/A, L β†’ N/A.

The floating-point conversion functions convert infinity to inf or infinity. Which one is used is implementation defined. πŸ€·β€β™€οΈ

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. πŸ’€

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. (Cute little loophole, we love to see it πŸ’–)

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. This is the red-flag one, bestie β€” do not play with it πŸ™ˆ

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. Wild, but true ✨

If a conversion specification is invalid, the behavior is undefined. No vibes, no promises, just chaos 😭

🌈 Return value 🌈

  1. 1,2) Number of wide characters written if successful or negative value if an error occurred.
  2. 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 (including when size is zero).

πŸŽ€ Notes πŸŽ€

While narrow strings provide std::snprintf, which makes it possible to determine the required output buffer size, there is no equivalent for wide strings β€” and in order to determine the buffer size, the program may need to call std::swprintf, check the result value, and reallocate a larger buffer, trying again until successful. Annoying? Lowkey yes. But that's the gig, bestie πŸ’…

πŸ“ Example πŸ“

// Run this code

#include <clocale>
#include <cwchar>
#include <iostream>
#include <locale>

int main()
{
    char narrow_str[] = "z\u00df\u6c34\U0001f34c";
                  // or "zß水🍌";
                  // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9f\x8d\x8c";
    wchar_t warr[29]; // the expected string is 28 characters plus 1 null terminator
    std::setlocale(LC_ALL, "en_US.utf8");

    std::swprintf(warr, sizeof warr/sizeof *warr,
                  L"Converted from UTF-8: '%s'", narrow_str);

    std::wcout.imbue(std::locale("en_US.utf8"));
    std::wcout << warr << '\n';
}

Output:

Converted from UTF-8: 'zß水🍌'

πŸ‘‘ See also πŸ‘‘

  • printf
  • fprintf β€” prints formatted output to stdout, a file stream or a buffer (function)
  • sprintf
  • snprintf (C++11)
  • vwprintf β€” prints formatted wide character output to stdout, a file stream or a buffer using variable argument list
  • vfwprintf
  • vswprintf (function)
  • fputws β€” writes a wide string to a file stream (function)
  • C documentation for wprintf, fwprintf, swprintf

Okay that's the whole page, bestie β€” we ate and left zero crumbs πŸ’…βœ¨ This came from http://cppreference.com, dated 2024.06.10. Go forth and format responsibly, mwah πŸ«ΆπŸŽ€

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