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

int ilogb ( float arg ); int ilogbf( float arg ); | (1) | (since C++11) |

int ilogb ( double arg ); | (2) | (since C++11) |

int ilogb ( long double arg ); int ilogbl( long double arg ); | (3) | (since C++11) |

int ilogb ( IntegralType arg ); | (4) | (since C++11) |

#define FP_ILOGB0 /*implementation-defined*/ | (5) | (since C++11) |

#define FP_ILOGBNAN /*implementation-defined*/ | (6) | (since C++11) |

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

`arg`

, and returns it as a signed integer 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 integral part of log

r|arg| as a signed integral value, for non-zero `arg`

, where `r`

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

and `T`

is the floating-point type of `arg`

.

arg | - | floating point value |

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

is returned as a signed int value.

If `arg`

is zero, `FP_ILOGB0`

is returned.

If `arg`

is infinite, `INT_MAX`

is returned.

If `arg`

is a NaN, `FP_ILOGBNAN`

is returned.

If the correct result is greater than `INT_MAX`

or smaller than `INT_MIN`

, the return value is unspecified.

Errors are reported as specified in `math_errhandling`

.

A domain error or range error may occur if `arg`

is zero, infinite, or NaN.

If the correct result is greater than `INT_MAX`

or smaller than `INT_MIN`

, a domain error or a range error may occur.

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

- If the correct result is greater than
`INT_MAX`

or smaller than`INT_MIN`

,`FE_INVALID`

is raised. - If
`arg`

is ±0, ±∞, or NaN,`FE_INVALID`

is raised. - In all other cases, the result is exact (
`FE_INEXACT`

is never raised) and the current rounding mode is ignored

If `arg`

is not zero, infinite, or NaN, the value returned is exactly equivalent to `static_cast<int>(std::logb(arg))`

.

POSIX requires that a domain error occurs if `arg`

is zero, infinite, NaN, or if the correct result is outside of the range of `int`

.

POSIX also requires that, on XSI-conformant systems, the value returned when the correct result is greater than `INT_MAX`

is `INT_MAX`

and the value returned when the correct result is less than `INT_MIN`

is `INT_MIN`

.

The correct result can be represented as `int`

on all known implementations. For overflow to occur, `INT_MAX`

must be less than `LDBL_MAX_EXP*log2(FLT_RADIX)`

or `INT_MIN`

must be greater than `LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX)`

.

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

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

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

returned by `std::ilogb`

, |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 << "ilogb(0) = " << std::ilogb(0) << '\n'; if (std::fetestexcept(FE_INVALID)) std::cout << " FE_INVALID 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 ilogb(0) = -2147483648 FE_INVALID 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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