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
(C++11)
bool (C++11) 0
char (C++11) 0
signed char (C++11) 0
unsigned char (C++11) 0
wchar_t (C++11) 0
char8_t (C++20) 0
char16_t (C++11) 0
char32_t (C++11) 0
short (C++11) 0
unsigned short (C++11) 0
int (C++11) 0
unsigned int (C++11) 0
long (C++11) 0
unsigned long (C++11) 0
long long (C++11) 0
unsigned long long 0
(C++11)
FLT_DECIMAL_DIG or
float (C++11) std::ceil(std::numeric_limits<float>::digits * std::log10(2)
+ 1)
DBL_DECIMAL_DIG
double (C++11) orstd::ceil(std::numeric_limits<double>::digits *
std::log10(2) + 1)
DECIMAL_DIG or LDBL_DECIMAL_DIG
long double (C++11) orstd::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

// Run this code

#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