|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Basic types | |||||||||||||||||||||
| Fixed width integer types(C++11) | |||||||||||||||||||||
| Fixed width floating-point types(C++23) | |||||||||||||||||||||
| |||||||||||||||||||||
| Numeric limits | |||||||||||||||||||||
| C numeric limits interface | |||||||||||||||||||||
| Runtime type information | |||||||||||||||||||||
| |||||||||||||||||||||
| Static constants | ||||
numeric_limits::max_digits10 (C++11) | ||||
| Static member functions | ||||
(C++11) | ||||
| Helper types | ||||
staticconstexprint max_digits10 | (since C++11) | |
The value ofstd::numeric_limits<T>::max_digits10 is the number of base-10 digits that are necessary to uniquely represent all distinct values of the typeT, such as necessary for serialization/deserialization to text. This constant is meaningful for all floating-point types.
Contents |
T | Value ofstd::numeric_limits<T>::max_digits10 |
| /* non-specialized */ | 0 |
| bool | 0 |
| char | 0 |
| signedchar | 0 |
| unsignedchar | 0 |
| wchar_t | 0 |
| char8_t(since C++20) | 0 |
| char16_t | 0 |
| char32_t | 0 |
| short | 0 |
| unsignedshort | 0 |
| int | 0 |
| unsignedint | 0 |
| long | 0 |
| unsignedlong | 0 |
| longlong | 0 |
| unsignedlonglong | 0 |
| float | FLT_DECIMAL_DIG orstd::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) |
| longdouble | DECIMAL_DIG orLDBL_DECIMAL_DIG orstd::ceil(std::numeric_limits<longdouble>::digits*std::log10(2)+1) |
Unlike most mathematical operations, the conversion of a floating-point value to text and back isexact as long as at leastmax_digits10 were used (9 forfloat,17 fordouble): 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 afloat in decimal notation.
#include <cmath>#include <iomanip>#include <iostream>#include <limits>#include <sstream> int main(){float value=10.0000086; constexprauto digits10=std::numeric_limits<decltype(value)>::digits10;constexprauto max_digits10=std::numeric_limits<decltype(value)>::max_digits10;constexprauto 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"; constauto 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 digitssubmax_digits10 is 8 digits max_digits10: 10.0000086submax_digits10: 10.000009 max_digits10: 10.0000095submax_digits10: 10.00001 max_digits10: 10.0000105submax_digits10: 10.00001 max_digits10: 10.0000114submax_digits10: 10.000011 max_digits10: 10.0000124submax_digits10: 10.000012
[static] | the radix or integer base used by the representation of the given type (public static member constant)[edit] |
[static] | number ofradix 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] |
[static] | one more than the smallest negative power of the radix that is a valid normalized floating-point value (public static member constant)[edit] |
[static] | one more than the largest integer power of the radix that is a valid finite floating-point value (public static member constant)[edit] |
