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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../ranges/../io/c/vfprintf.html below:

std::vprintf, std::vfprintf, std::vsprintf, std::vsnprintf - cppreference.com

int vprintf( const char* format, std::va_list vlist );

int vsprintf( char* buffer, const char* format, std::va_list vlist );

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

Writes the results to

.

2) Writes the results to a file stream stream.

3) Writes the results to a character string buffer.

4) Writes the results to a character string buffer. At most buf_size - 1 characters are written. The resulting character string will be terminated with a null character, unless buf_size is zero. If buf_size is zero, nothing is written and buffer may be a null pointer, however the return value (number of bytes that would be written not including the null terminator) is still calculated and returned.

The format string consists of ordinary byte 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:

Writes a single character.

• The argument is first converted to unsigned char.
• If the l modifier is used, the argument is first converted to a character string as if by %ls with a wchar_t[2] argument.

int

std::wint_t

Writes a character string.

• The argument must be a pointer to the initial element of an array of characters.
• Precision specifies the maximum number of bytes to be written. If Precision is not specified, writes every byte 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, which is converted to char array as if by a call to std::wcrtomb with zero-initialized conversion state.

char*

wchar_t*

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.
• For the z modifier, the expected argument type is the signed version of std::size_t.

signed char

short

int

long

long long

※

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.

unsigned char

unsigned short

unsigned int

unsigned long

unsigned long long

unsigned version of

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.

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.

Converts floating-point number to the decimal notation in the style [-]ddd.ddd.

• Precision specifies the exact 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.

double

double (C++11)

long double

Converts floating-point number to the decimal exponent notation.

• For the e conversion style [-]d.ddd e±dd is used.
• For the E conversion style [-]d.ddd E±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 decimal point character is written even if no digits follow it.
• For infinity and not-a-number conversion style see notes.

(C++11)

Converts floating-point number to the hexadecimal exponent notation.

• For the a conversion style [-] 0xh.hhh p±d is used.
• For the A conversion style [-] 0Xh.hhh P±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 decimal point character is written even if no digits follow it.
• For infinity and not-a-number conversion style see notes.

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(until C++11)F(since C++11) 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(since C++11) 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.

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.
• For the z modifier, the expected argument type is S*, where S is the signed version of std::size_t.

signed char*

short*

int*

long*

long long*

※

Writes an implementation defined character sequence defining a pointer.

void*

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.

The conversion specifier used to print char, unsigned char, signed char, short, and unsigned short expects promoted types of default argument promotions, but before printing its value will be converted to char, unsigned char, signed char, short, and unsigned short. It is safe to pass values of these types because of the promotion that takes place when a variadic function is called.

The correct conversion specifications for the fixed-width character types (std::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.

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.

1-3) Number of characters written if successful or negative value if an error occurred.

4) Number of characters written if successful or negative value if an error occurred. If the resulting string gets truncated due to buf_size limit, function returns the total number of characters (not including the terminating null-byte) which would have been written, if the limit was not imposed.

All these functions invoke va_arg at least once, the value of arg is indeterminate after the return. These functions do not invoke va_end, and it must be done by the caller.

#include <cstdarg>
#include <cstdio>
#include <ctime>
#include <vector>
 
void debug_log(const char *fmt, ...)
{
    std::time_t t = std::time(nullptr);
    char time_buf[100];
    std::strftime(time_buf, sizeof time_buf, "%D %T", std::gmtime(&t));
    std::va_list args1;
    va_start(args1, fmt);
    std::va_list args2;
    va_copy(args2, args1);
    std::vector<char> buf(1 + std::vsnprintf(nullptr, 0, fmt, args1));
    va_end(args1);
    std::vsnprintf(buf.data(), buf.size(), fmt, args2);
    va_end(args2);
    std::printf("%s [debug]: %s\n", time_buf, buf.data());
}
 
int main()
{
    debug_log("Logging, %d, %d, %d", 1, 2, 3);
}

Output:

04/13/15 15:09:18 [debug]: Logging, 1, 2, 3

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