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std::numeri...ts::digits10(3)  C++ Standard Libary std::numeri...ts::digits10(3)

NAME
     std::numeric_limits::digits10 - std::numeric_limits::digits10

Synopsis
	static const int digits10;	(until C++11)
	static constexpr int digits10;	(since C++11)

	The  value  of std::numeric_limits<T>::digits10 is the number of base-10
     digits that
	can be represented by the type T without change,  that	is,  any  number
     with this many
	significant  decimal  digits  can  be converted to a value of type T and
     back to decimal
	form, without change due to rounding or overflow. For base-radix  types,
     it is the
	value  of  digits()  (digits - 1 for floating-point types) multiplied by
     \(\small
	\log_{10}{radix}\)log
	10(radix) and rounded down.

Standard specializations
	T			  value of std::numeric_limits<T>::digits10
	/* non-specialized */	  0
	bool			  0
	char			       std::numeric_limits<char>::digits       *
     std::log10(2)
	signed	char		    std::numeric_limits<signed	char>::digits  *
     std::log10(2)
	unsigned char		  std::numeric_limits<unsigned	char>::digits  *
     std::log10(2)
	wchar_t 		      std::numeric_limits<wchar_t>::digits     *
     std::log10(2)
	char8_t   (since   C++20)	std::numeric_limits<char8_t>::digits   *
     std::log10(2)
	char16_t   (since   C++11)	std::numeric_limits<char16_t>::digits  *
     std::log10(2)
	char32_t  (since   C++11)      std::numeric_limits<char32_t>::digits   *
     std::log10(2)
	short			       std::numeric_limits<short>::digits      *
     std::log10(2)
	unsigned short		  std::numeric_limits<unsigned short>::digits *
				  std::log10(2)
	int				std::numeric_limits<int>::digits       *
     std::log10(2)
	unsigned  int		    std::numeric_limits<unsigned  int>::digits *
     std::log10(2)
	long			       std::numeric_limits<long>::digits       *
     std::log10(2)
	unsigned  long		    std::numeric_limits<unsigned long>::digits *
     std::log10(2)
	long  long  (since  C++11)    std::numeric_limits<long	long>::digits  *
     std::log10(2)
	unsigned  long long (since std::numeric_limits<unsigned long long>::dig-
     its *
	C++11)			  std::log10(2)
	float			  FLT_DIG (6 for IEEE float)
	double			  DBL_DIG (15 for IEEE double)
	long double		  LDBL_DIG (18 for 80-bit Intel long double;  33
     for IEEE
				  quadruple)

Example
	An 8-bit binary type can represent any two-digit decimal number exactly,
     but 3-digit
	decimal  numbers  256..999  cannot be represented. The value of digits10
     for an 8-bit
	type is 2 (8 * std::log10(2) is 2.41)

	The standard 32-bit IEEE 754 floating-point type has a 24 bit fractional
     part (23
	bits written, one implied), which may suggest that it  can  represent  7
     digit decimals
	(24  * std::log10(2) is 7.22), but relative rounding errors are non-uni-
     form and some
	floating-point values with 7 decimal digits do not survive conversion to
     32-bit
	float and back: the smallest positive example is 8.589973e9,  which  be-
     comes
	8.589974e9  after the roundtrip. These rounding errors cannot exceed one
     bit in the
	representation, and digits10 is calculated as (24 - 1) *  std::log10(2),
     which is
	6.92. Rounding down results in the value 6.

	Likewise,   the   16-digit  string  9007199254740993  does  not  survive
     text->double->text
	roundtrip, becoming 9007199254740992: the 64-bit IEEE  754  type  double
     guarantees
	this roundtrip only for 15 decimal digits.

See also
	max_digits10	 number of decimal digits necessary to differentiate all
     values of
	[static] (C++11) this type
			 (public static member constant)
	radix		 the radix or integer base used by the representation of
     the given
	[static]	 type
			 (public static member constant)
	digits		  number of radix digits that can be represented without
     change
	[static]	 (public static member constant)
	min_exponent	 one more than the smallest negative power of the  radix
     that is a
	[static]	 valid normalized floating-point value
			 (public static member constant)
	max_exponent	  one  more  than the largest integer power of the radix
     that is a
	[static]	 valid finite floating-point value
			 (public static member constant)

http://cppreference.com 	   2024.06.10	   std::numeri...ts::digits10(3)
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