# std::log, std::logf, std::logl

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< cpp‎ | numeric‎ | math

Defined in header `<cmath>` |  |
| (1) |
float log ( float num ); double log ( double num ); long double log ( long double num ); |  | (until C++23)
/* floating-point-type */ log ( /* floating-point-type */ num ); |  | (since C++23) (constexpr since C++26)
float logf( float num ); | (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 > double log ( Integer num ); | (A) | (constexpr since C++26)

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 treated as double. | (since C++11)

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

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