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Showing content from https://en.cppreference.com/w/cpp/language/../numeric/math/log2.html below:

std::log2, std::log2f, std::log2l - cppreference.com

(1) float       log2 ( float num );

double      log2 ( double num );

long double log2 ( long double num ); (until C++23)

/*floating-point-type*/
            log2 ( /*floating-point-type*/ num );

(since C++23)
(constexpr since C++26)

float       log2f( float num );

(2) (since C++11)
(constexpr since C++26)

long double log2l( long double num );

(3) (since C++11)
(constexpr since C++26) template< /*math-floating-point*/ V >

constexpr /*deduced-simd-t*/<V>

            log2 ( const V& v_num ); (S) (since C++26)

template< class Integer >
double      log2 ( Integer num );

(A) (constexpr since C++26) 1-3)

Computes the

binary (base-2) logarithm

of

num

.

The library provides overloads of std::log2 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) [edit] Parameters num - floating-point or integer value [edit] Return value

If no errors occur, the base-2 logarithm of num (log2(num) or lb(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.

[edit] 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),

[edit] Notes

For integer num, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.

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::log2(num) has the same effect as std::log2(static_cast<double>(num)).

[edit] Example 
#include <cerrno>
#include <cfenv>
#include <cmath>
#include <cstring>
#include <iostream>
// #pragma STDC FENV_ACCESS ON
 
int main()
{
    std::cout << "log2(65536) = " << std::log2(65536) << '\n'
              << "log2(0.125) = " << std::log2(0.125) << '\n'
              << "log2(0x020f) = " << std::log2(0x020f)
              << " (highest set bit is in position 9)\n"
              << "base-5 logarithm of 125 = "
              << std::log2(125) / std::log2(5) << '\n';
 
    // special values
    std::cout << "log2(1) = " << std::log2(1) << '\n'
              << "log2(+Inf) = " << std::log2(INFINITY) << '\n';
 
    // error handling
    errno = 0;
    std::feclearexcept(FE_ALL_EXCEPT);
 
    std::cout << "log2(0) = " << std::log2(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:

log2(65536) = 16
log2(0.125) = -3
log2(0x020f) = 9.04166 (highest set bit is in position 9)
base-5 logarithm of 125 = 3
log2(1) = 0
log2(+Inf) = inf
log2(0) = -inf
    errno == ERANGE: Numerical result out of range
    FE_DIVBYZERO raised
[edit] See also computes natural (base e) logarithm (\({\small\ln{x}}\)ln(x))
(function) [edit] computes common (base 10) logarithm (\({\small\log_{10}{x}}\)log10(x))
(function) [edit] natural logarithm (to base e) of 1 plus the given number (\({\small\ln{(1+x)}}\)ln(1+x))
(function) [edit] returns 2 raised to the given power (\({\small 2^x}\)2x)
(function) [edit]

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