Log

Defined in header <cmath>.

Description

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 num.

Declarations

C++23

// 1)
/* floating-point-type */ log( /* floating-point-type */ num );
// 2)
float logf( float num );
// 3)
long double logl( long double num );

Additional Overloads

// 4)
template< class Integer >
double log ( Integer num );

C++11

// 1)
float log ( float num );
// 2)
double log ( double num );
// 3)
long double log ( long double num );
// 4)
float logf( float num );
// 5)
long double logl( long double num );

Additional Overloads

// 6)
template< class Integer >
double log ( Integer num );

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 Additional Overloads. 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)).

Examples

#include <cerrno>
#include <cfenv>
#include <cstring>
#include <cmath>
#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 Result

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