std::bit_floor

 

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Bit manipulation

bit_cast (C++20)

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Integral powers of 2

has_single_bit (C++20)

bit_ceil (C++20)

bit_floor (C++20)

bit_width (C++20)

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Defined in header <bit>
template< class T > constexpr T bit_floor( T x ) noexcept;		(since C++20)

If x is not zero, calculates the largest integral power of two that is not greater than x. If x is zero, returns zero.

Parameters

x	-	a value to compare with
Type requirements
T	-	must be an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type) in order to participate in overload resolution.

Return value

Zero if x is zero; otherwise, the largest integral power of two that is not greater than x.

Notes

Prior to P1956R1, the proposed name for this function template was floor2.

Feature-test macro	Value	Std	Feature
__cpp_lib_int_pow2	202002L	(C++20)	Integral power-of-2 operations

Possible implementation

template<typename T, typename ... U> concept neither = (!std::same_as<T, U> && ...); template<std::unsigned_integral T> requires neither<T, bool, char, char8_t, char16_t, char32_t, wchar_t> constexpr T bit_floor(T x) noexcept { if (x != 0) return T{1} << (std::bit_width(x) - 1); return 0; }

Example

Run this code

#include <bit>
#include <cstdio>
#include <print>

int main()
{
    for (unsigned x{}; x != 9; ++x)
    {
        if (std::has_single_bit(x))
            std::putchar('\n');

        std::print("bit_floor({:04b}) = {:04b}\n", x, std::bit_floor(x));
    }
}

Output:

bit_floor(0000) = 0000

bit_floor(0001) = 0001

bit_floor(0010) = 0010
bit_floor(0011) = 0010

bit_floor(0100) = 0100
bit_floor(0101) = 0100
bit_floor(0110) = 0100
bit_floor(0111) = 0100

bit_floor(1000) = 1000

See also

bit_ceil (C++20)	finds the smallest integral power of 2 not less than the given value (function template)
rotr (C++20)	computes the result of bitwise right-rotation (function template)
bit_width (C++20)	finds the smallest number of bits needed to represent the given value (function template)
has_single_bit (C++20)	checks if a number is an integral power of 2 (function template)