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std::numeric_limits::max_digits10

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` or`std::ceil(std::numeric_limits<double>::digits * std::log10(2) + 1`)
`long double`	`DECIMAL_DIG` or `LDBL_DECIMAL_DIG` or`std::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.

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)

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std::numeric_limits::max_digits10