std::numeric_limits<T>::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`
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` 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)`
longdouble	`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.

Example

#include <cmath>
#include <iomanip>
#include <iostream>
#include <limits>
#include <sstream>

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

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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https://en.cppreference.com/cpp/types/numeric_limits/max_digits10

std::numeric_limits<T>::max_digits10

Standard specializations

Notes

Example

See also