Defined in header `<cmath>` | ||
---|---|---|

float logb ( float arg ); float logbf( float arg ); | (1) | (since C++11) |

double logb ( double arg ); | (2) | (since C++11) |

long double logb ( long double arg ); long double logbl( long double arg ); | (3) | (since C++11) |

double logb ( IntegralType arg ); | (4) | (since C++11) |

1-3) Extracts the value of the unbiased radix-independent exponent from the floating-point argument

`arg`

, and returns it as a floating-point value.
4) A set of overloads or a function template accepting an argument of any integral type. Equivalent to (2) (the argument is cast to

`double`

).Formally, the unbiased exponent is the signed integral part of log

r|arg| (returned by this function as a floating-point value), for non-zero arg, where `r`

is `std::numeric_limits<T>::radix`

and `T`

is the floating-point type of `arg`

. If `arg`

is subnormal, it is treated as though it was normalized.

arg | - | floating point value |

If no errors occur, the unbiased exponent of `arg`

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.

Errors are reported as specified in `math_errhandling`

.

Domain or range error may occur if `arg`

is zero.

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

- If
`arg`

is ±0, -∞ is returned and`FE_DIVBYZERO`

is raised. - If
`arg`

is ±∞, +∞ is returned - If
`arg`

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

POSIX requires that a pole error occurs if `arg`

is ±0.

The value of the exponent returned by `std::logb`

is always 1 less than the exponent retuned by `std::frexp`

because of the different normalization requirements: for the exponent `e`

returned by `std::logb`

, |arg*r-e

| is between 1 and `r`

(typically between `1`

and `2`

), but for the exponent `e`

returned by `std::frexp`

, |arg*2-e

| is between `0.5`

and `1`

.

Compares different floating-point decomposition functions.

#include <iostream> #include <cmath> #include <limits> #include <cfenv> #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

(C++11)(C++11) | decomposes a number into significand and a power of `2` (function) |

(C++11)(C++11)(C++11) | extracts exponent of the number (function) |

(C++11)(C++11)(C++11)(C++11)(C++11)(C++11) | multiplies a number by `FLT_RADIX` raised to a power (function) |

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