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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`
`signed char`	`0`
`unsigned char`	`0`
`wchar_t`	`0`
`char8_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`	`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)`

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 <limits>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <iostream>
 
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 );
 
    return 0;
}

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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http://en.cppreference.com/w/cpp/types/numeric_limits/max_digits10

std::numeric_limits<T>::max_digits10

Standard specializations

Notes

Example

See also