NAME¶

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

Synopsis¶

Defined in header <cmath>
float logb ( float num );

double logb ( double num ); (until C++23)

long double logb ( long double num );
constexpr /* floating-point-type */ (since C++23)
logb ( /* floating-point-type */ num );
float logbf( float num ); (1) (2) (since C++11)
(constexpr since C++23)
long double logbl( long double num ); (3) (since C++11)
(constexpr since C++23)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer > (A) (constexpr since C++23)
double logb ( Integer num );

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)

A) Additional overloads are provided for all integer types, which are (since C++11)
treated as double.

Formally, the unbiased exponent is the signed integral part of log
r|num| (returned by this function as a floating-point value), for non-zero num,
where r is std::numeric_limits<T>::radix and T is the floating-point type of num. If
num is subnormal, it is treated as though it was normalized.

Parameters¶

num - floating-point or integer value

Return value¶

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

If a domain error occurs, an implementation-defined value is returned.

If a pole error occurs, -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL is returned.

Error handling¶

Errors are reported as specified in math_errhandling.

Domain or range error may occur if num is zero.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

* If num is ±0, -∞ is returned and FE_DIVBYZERO is raised.
* If num is ±∞, +∞ is returned.
* If num is NaN, NaN is returned.
* In all other cases, the result is exact (FE_INEXACT is never raised) and the
current rounding mode is ignored.

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, |num*r-e
| is between 1 and r (typically between 1 and 2), but for the exponent e returned by
std::frexp, |num*2-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<double>(num)).

Example¶

Compares different floating-point decomposition functions:

// Run this code

#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";
}

Possible output:¶

Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
modf() makes 123 + 0.45
frexp() makes 0.964453 * 2^7
logb()/ilogb() make 1.92891 * 2^6
logb(0) = -Inf
FE_DIVBYZERO raised

See also¶

frexp
frexpf decomposes a number into significand and base-2 exponent
frexpl (function)
(C++11)
(C++11)
ilogb
ilogbf
ilogbl extracts exponent of the number
(C++11) (function)
(C++11)
(C++11)
scalbn
scalbnf
scalbnl
scalbln
scalblnf
scalblnl multiplies a number by FLT_RADIX raised to a power
(C++11) (function)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
C documentation for
logb