std::bit_floor

 

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Bit manipulation

bit_cast (C++20)

byteswap (C++23)

Integral powers of 2

has_single_bit (C++20)

bit_ceil (C++20)

bit_floor (C++20)

bit_width (C++20)

Rotating

rotl (C++20)

rotr (C++20)

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countl_zero (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.

This overload participates in overload resolution only if T is an unsigned integer type (that is, unsignedchar, unsignedshort, unsignedint, unsignedlong, unsignedlonglong, or an extended unsigned integer type).

Parameters

x

-

unsigned integer value

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);return0;}

Example

Run this code

#include <bit>#include <bitset>#include <iostream>   int main(){using bin =std::bitset<8>;for(unsigned x{}; x !=012;++x)std::cout<<"bit_floor( "<< bin(x)<<" ) = "<< bin(std::bit_floor(x))<<'\n';}

Output:

bit_floor( 00000000 ) = 00000000 bit_floor( 00000001 ) = 00000001 bit_floor( 00000010 ) = 00000010 bit_floor( 00000011 ) = 00000010 bit_floor( 00000100 ) = 00000100 bit_floor( 00000101 ) = 00000100 bit_floor( 00000110 ) = 00000100 bit_floor( 00000111 ) = 00000100 bit_floor( 00001000 ) = 00001000 bit_floor( 00001001 ) = 00001000

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)