From cppreference.com
Defined in header
int exponent
floatscalbn(floatnum,intexp);doublescalbn(doublenum,intexp);longdoublescalbn(longdoublenum,intexp); (1)(since C++11)
(until C++23)constexpr/* floating-point-type */scalbn(/* floating-point-type */num,intexp);(since C++23)floatscalbnf(floatnum,intexp); (2) (since C++11)
(constexpr since C++23)longdoublescalbnl(longdoublenum,intexp); (3) (since C++11)
(constexpr since C++23)long exponent
floatscalbln(floatnum,longexp);doublescalbln(doublenum,longexp);longdoublescalbln(longdoublenum,longexp); (4)(since C++11)
(until C++23)constexpr/* floating-point-type */scalbln(/* floating-point-type */num,longexp);(since C++23)floatscalblnf(floatnum,longexp); (5) (since C++11)
(constexpr since C++23)longdoublescalblnl(longdoublenum,longexp); (6) (since C++11)
(constexpr since C++23)
Defined in header
template<classInteger>doublescalbn(Integernum,intexp); (A) (since C++11)
(constexpr since C++23)template<classInteger>doublescalbln(Integernum,longexp); (B) (since C++11)
(constexpr since C++23)1-6) Multiplies a floating point value num by
raised to power exp. The library provides overloads of std::scalbn and std::scalbln for all cv-unqualified floating-point types as the type of the parameter num.(since C++23)
A,B) Additional overloads are provided for all integer types, which are treated as double.
Parameters
num - floating-point or integer value exp - integer value Return value
If no errors occur, num multiplied by
to the power of exp (num×FLT_RADIXexp
) is returned.
If a range error due to overflow occurs,
, ±HUGE_VALF, or ±HUGE_VALL is returned.
If a range error due to underflow occurs, the correct result (after rounding) is returned.
Error handling
Errors are reported as specified in
.
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
Unless a range error occurs,
is never raised (the result is exact).
Unless a range error occurs,
is ignored.
If num is ±0, it is returned, unmodified.
If num is ±∞, it is returned, unmodified.
If exp is 0, then num is returned, unmodified.
If num is NaN, NaN is returned.
Notes
On binary systems (where
is 2), std::scalbn is equivalent to
.
Although std::scalbn and std::scalbln are specified to perform the operation efficiently, on many implementations they are less efficient than multiplication or division by a power of two using arithmetic operators.
The function name stands for "new scalb", where scalb was an older non-standard function whose second argument had floating-point type.
The std::scalbln function is provided because the factor required to scale from the smallest positive floating-point value to the largest finite one may be greater than 32767, the standard-guaranteed
. In particular, for the 80-bit longdouble, the factor is 32828.
The GNU implementation does not set errno regardless of math_errhandling.
The additional overloads are not required to be provided exactly as (A,B). They only need to be sufficient to ensure that for their argument num of integer type:
std::scalbn(num,exp) has the same effect as std::scalbn(static_cast<double>(num),exp).
std::scalbln(num,exp) has the same effect as std::scalbln(static_cast<double>(num),exp).
Example
Run this code
#include<cerrno>#include<cfenv>#include<cmath>#include<cstring>#include<iostream>// #pragma STDC FENV_ACCESS ONintmain(){std::cout<<"scalbn(7, -4) = "<<std::scalbn(7,-4)<<'\n'<<"scalbn(1, -1074) = "<<std::scalbn(1,-1074)<<" (minimum positive subnormal double)\n"<<"scalbn(nextafter(1,0), 1024) = "<<std::scalbn(std::nextafter(1,0),1024)<<" (largest finite double)\n";// special valuesstd::cout<<"scalbn(-0, 10) = "<<std::scalbn(-0.0,10)<<'\n'<<"scalbn(-Inf, -1) = "<<std::scalbn(-INFINITY,-1)<<'\n';// error handlingerrno=0;std::feclearexcept(FE_ALL_EXCEPT);std::cout<<"scalbn(1, 1024) = "<<std::scalbn(1,1024)<<'\n';if(errno==ERANGE)std::cout<<" errno == ERANGE: "<<std::strerror(errno)<<'\n';if(std::fetestexcept(FE_OVERFLOW))std::cout<<" FE_OVERFLOW raised\n";}Possible output:
scalbn(7, -4) = 0.4375 scalbn(1, -1074) = 4.94066e-324 (minimum positive subnormal double) scalbn(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double) scalbn(-0, 10) = -0 scalbn(-Inf, -1) = -inf scalbn(1, 1024) = inf errno == ERANGE: Numerical result out of range FE_OVERFLOW raised See also