NAME¶

std::log,std::logf,std::logl - std::log,std::logf,std::logl

Synopsis¶

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

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

long double log ( long double num );
/* floating-point-type */ (since C++23)
log ( /* floating-point-type */ num ); (constexpr since C++26)
float logf( float num ); (1) (2) (since C++11)
(constexpr since C++26)
long double logl( long double num ); (3) (since C++11)
(constexpr since C++26)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer > (A) (constexpr since C++26)
double log ( Integer num );

1-3) Computes the natural (base e) logarithm of num.
The library provides overloads of std::log 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.

Parameters¶

num - floating-point or integer value

Return value¶

If no errors occur, the natural (base-e) logarithm of num (ln(num) or log
e(num)) is returned.

If a domain error occurs, an implementation-defined value is returned (NaN where
supported).

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 error occurs if num is less than zero.

Pole error may occur if num is zero.

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

* If the argument is ±0, -∞ is returned and FE_DIVBYZERO is raised.
* If the argument is 1, +0 is returned.
* If the argument is negative, NaN is returned and FE_INVALID is raised.
* If the argument is +∞, +∞ is returned.
* If the argument is NaN, NaN is returned.

Notes¶

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::log(num) has the same effect as std::log(static_cast<double>(num)).

Example¶

// Run this code

#include <cerrno>
#include <cfenv>
#include <cmath>
#include <cstring>
#include <iostream>
// #pragma STDC FENV_ACCESS ON

int main()
{
std::cout << "log(1) = " << std::log(1) << '\n'
<< "base-5 logarithm of 125 = " << std::log(125) / std::log(5) << '\n';

// special values
std::cout << "log(1) = " << std::log(1) << '\n'
<< "log(+Inf) = " << std::log(INFINITY) << '\n';

// error handling
errno = 0;
std::feclearexcept(FE_ALL_EXCEPT);

std::cout << "log(0) = " << std::log(0) << '\n';

if (errno == ERANGE)
std::cout << " errno == ERANGE: " << std::strerror(errno) << '\n';
if (std::fetestexcept(FE_DIVBYZERO))
std::cout << " FE_DIVBYZERO raised\n";
}

Possible output:¶

log(1) = 0
base-5 logarithm of 125 = 3
log(1) = 0
log(+Inf) = inf
log(0) = -inf
errno == ERANGE: Numerical result out of range
FE_DIVBYZERO raised

See also¶

log10
log10f computes common (base 10) logarithm
log10l (\({\small\log_{10}{x}}\)log[10](x))
(C++11) (function)
(C++11)
log2
log2f base 2 logarithm of the given number
log2l (\({\small\log_{2}{x}}\)log[2](x))
(C++11) (function)
(C++11)
(C++11)
log1p
log1pf natural logarithm (to base e) of 1 plus the given number
log1pl (\({\small\ln{(1+x)}}\)ln(1+x))
(C++11) (function)
(C++11)
(C++11)
exp
expf returns e raised to the given power (\({\small e^x}\)e^x)
expl (function)
(C++11)
(C++11)
complex natural logarithm with the branch cuts along the negative
log(std::complex) real axis
(function template)
log(std::valarray) applies the function std::log to each element of valarray
(function template)
C documentation for
log