std::numeric_limits::max_digits10 - cppreference.com (original) (raw)
| 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 |
| char8_t (since C++20) | 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) |
[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] Example
#include #include #include #include #include int main() { float value = 10.0000086; constexpr auto digits10 = std::numeric_limits<decltype(value)>::digits10; constexpr auto max_digits10 = std::numeric_limits<decltype(value)>::max_digits10; constexpr auto submax_digits10 = max_digits10 - 1; std::cout << "float:\n" " digits10 is " << digits10 << " digits\n" " max_digits10 is " << max_digits10 << " digits\n" "submax_digits10 is " << submax_digits10 << " digits\n\n"; const auto original_precision = std::cout.precision(); for (auto i = 0; i < 5; ++i) { std::cout << " max_digits10: " << std::setprecision(max_digits10) << value << "\n" "submax_digits10: " << std::setprecision(submax_digits10) << value << "\n\n"; value = std::nextafter(value, std::numeric_limits<decltype(value)>::max()); } std::cout.precision(original_precision); }
Output:
float: digits10 is 6 digits max_digits10 is 9 digits submax_digits10 is 8 digits max_digits10: 10.0000086 submax_digits10: 10.000009 max_digits10: 10.0000095 submax_digits10: 10.00001 max_digits10: 10.0000105 submax_digits10: 10.00001 max_digits10: 10.0000114 submax_digits10: 10.000011 max_digits10: 10.0000124 submax_digits10: 10.000012
[edit] See also
| | the radix or integer base used by the representation of the given type (public static member constant) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | number of radix digits that can be represented without change (public static member constant) [edit] | | | 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] |