std::numeri...max_digits10(3) C++ Standard Libary std::numeri...max_digits10(3) NAME std::numeric_limits::max_digits10 - std::numeric_limits::max_digits10 Synopsis 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 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 FLT_DECIMAL_DIG or float std::ceil(std::numeric_limits<float>::digits * std::log10(2) + 1) DBL_DECIMAL_DIG or double std::ceil(std::numeric_limits<double>::digits * std::log10(2) + 1) DECIMAL_DIG or LDBL_DECIMAL_DIG or long double 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 no- tation. Example // Run this code #include <cmath> #include <iomanip> #include <iostream> #include <limits> #include <sstream> int main() { float value = 10.0000086; constexpr auto digits10 = std::numeric_limits<decltype(value)>::dig- its10; constexpr auto max_digits10 = std::numeric_limits<de- cltype(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 << " dig- its\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<de- cltype(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 See also radix the radix or integer base used by the representation of the given type [static] (public static member constant) digits number of radix digits that can be represented without change [static] (public static member constant) digits10 number of decimal 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 valid [static] normalized floating-point value (public static member constant) max_exponent one more than the largest integer power of the radix that is a valid [static] finite floating-point value (public static member constant) http://cppreference.com 2024.06.10 std::numeri...max_digits10(3)
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