std::numeric_limits::max_digits10

 

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numeric_limits::digits10

numeric_limits::max_digits10 (C++11)

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numeric_limits::min_exponent10

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numeric_limits::max_exponent10

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static constexpr int max_digits10		(since C++11)

The value of std::numeric_limits<T>::max_digits10 is the number of base-10 digits that are necessary to uniquely represent all distinct values of the type T, such as necessary for serialization/deserialization to text. This constant is meaningful for all floating-point types.

Standard specializations

T	value of std::numeric_limits<T>::max_digits10
/* non-specialized */	​0​
bool	​0​
char	​0​
signed char	​0​
unsigned char	​0​
wchar_t	​0​
char16_t	​0​
char32_t	​0​
short	​0​
unsigned short	​0​
int	​0​
unsigned int	​0​
long	​0​
unsigned long	​0​
long long	​0​
unsigned long long	​0​
float	FLT_DECIMAL_DIG or std::ceil(std::numeric_limits<float>::digits * std::log10(2) + 1)
double	DBL_DECIMAL_DIG orstd::ceil(std::numeric_limits<double>::digits * std::log10(2) + 1)
long double	DECIMAL_DIG or LDBL_DECIMAL_DIG orstd::ceil(std::numeric_limits<long double>::digits * std::log10(2) + 1)

Notes

Unlike most mathematical operations, the conversion of a floating-point value to text and back is exact as long as at least max_digits10 were used (9 for float, 17 for double): it is guaranteed to produce the same floating-point value, even though the intermediate text representation is not exact. It may take over a hundred decimal digits to represent the precise value of a float in decimal notation.

See also

radix [static]	the radix or integer base used by the representation of the given type (public static member constant)
digits [static]	number of radix digits that can be represented without change (public static member constant)
digits10 [static]	number of decimal digits that can be represented without change (public static member constant)
min_exponent [static]	one more than the smallest negative power of the radix that is a valid normalized floating-point value (public static member constant)
max_exponent [static]	one more than the largest integer power of the radix that is a valid finite floating-point value (public static member constant)