std::logb, std::logbf, std::logbl

Since C++23

Header: <cmath>

1-3) Extracts the value of the unbiased radix-independent exponent from the floating-point argument num, and returns it as a floating-point value.The library provides overloads of std::logb for all cv-unqualified floating-point types as the type of the parameter.(since C++23)

# Declarations

float logb ( float num );
double logb ( double num );
long double logb ( long double num );

(until C++23)

constexpr /*floating-point-type*/
logb ( /*floating-point-type*/ num );

(since C++23)

float logbf( float num );

(since C++11) (constexpr since C++23)

long double logbl( long double num );

(since C++11) (constexpr since C++23)

SIMD overload (since C++26)

template< /*math-floating-point*/ V >
constexpr /*deduced-simd-t*/<V>
logb ( const V& v_num );

(since C++26)

Additional overloads (since C++11)

template< class Integer >
double logb ( Integer num );

(constexpr since C++23)

# Parameters

# Return value

If no errors occur, the unbiased exponent of num is returned as a signed floating-point value.

# Notes

POSIX requires that a pole error occurs if num is ±0.

The value of the exponent returned by std::logb is always 1 less than the exponent returned by std::frexp because of the different normalization requirements: for the exponent e returned by std::logb, |numr-e| is between 1 and r (typically between 1 and 2), but for the exponent e returned by std::frexp, |num2-e| is between 0.5 and 1.

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::logb(num) has the same effect as std::logb(static_cast(num)).

# Example

#include <cfenv>
#include <cmath>
#include <iostream>
#include <limits>
// #pragma STDC FENV_ACCESS ON
 
int main()
{
    double f = 123.45;
    std::cout << "Given the number " << f << " or " << std::hexfloat
              << f << std::defaultfloat << " in hex,\n";
 
    double f3;
    double f2 = std::modf(f, &f3);
    std::cout << "modf() makes " << f3 << " + " << f2 << '\n';
 
    int i;
    f2 = std::frexp(f, &i);
    std::cout << "frexp() makes " << f2 << " * 2^" << i << '\n';
 
    i = std::ilogb(f);
    std::cout << "logb()/ilogb() make " << f / std::scalbn(1.0, i) << " * "
              << std::numeric_limits<double>::radix
              << "^" << std::ilogb(f) << '\n';
 
    // error handling
    std::feclearexcept(FE_ALL_EXCEPT);
 
    std::cout << "logb(0) = " << std::logb(0) << '\n';
    if (std::fetestexcept(FE_DIVBYZERO))
        std::cout << "    FE_DIVBYZERO raised\n";
}

# See also