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std::numeric_limits::max_digits10

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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.

[edit] 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 std::floor(std::numeric_limits<float>::digits * std::log10(2) + 2)
double std::floor(std::numeric_limits<double>::digits * std::log10(2) + 2)
long double std::floor(std::numeric_limits<long double>::digits * std::log10(2) + 2)

[edit] 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.

[edit] See also

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