std::bit_floor

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Numerics library

Common mathematical functions
Mathematical special functions (C++17)
Mathematical constants (C++20)
Basic linear algebra algorithms (C++26)
Floating-point environment (C++11)
Complex numbers
Numeric array (valarray)
Pseudo-random number generation
Factor operations
gcd (C++17)	lcm (C++17)
Interpolations
midpoint (C++20)	lerp (C++20)
Saturation arithmetic
add_sat (C++26) sub_sat (C++26) saturate_cast (C++26)	mul_sat (C++26) div_sat (C++26)
Generic numeric operations
iota (C++11) ranges::iota (C++23) accumulate inner_product adjacent_difference partial_sum	reduce (C++17) transform_reduce (C++17) inclusive_scan (C++17) exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Bit operations
has_single_bit (C++20)     bit_cast (C++20) bit_ceil (C++20) bit_floor (C++20) bit_width (C++20) rotl (C++20) rotr (C++20)	popcount (C++20) countl_zero (C++20) countl_one (C++20) countr_zero (C++20) countr_one (C++20) byteswap (C++23) endian (C++20)

 

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, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, 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

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

Possible implementation

template <std::unsigned_integral T> requires !std::same_as<T, bool> && !std::same_as<T, char> && !std::same_as<T, char8_t> && !std::same_as<T, char16_t> && !std::same_as<T, char32_t> && !std::same_as<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 <bitset>
#include <iostream>
 
int main()
{
    using bin = std::bitset<8>;
 
    for (unsigned x = 0; x != 10; ++x)
    {
        auto const z = std::bit_floor(x); // `floor2` before P1956R1
 
        std::cout << "bit_floor( " << bin(x) << " ) = " << bin(z) << '\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 two 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)