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std::bit_floor

From cppreference.com
< cpp‎ | numeric
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 only participates in overload resolution 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).

Contents

[edit] Return value

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

[edit] 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;
}

[edit] Example

#include <bit>
#include <bitset>
#include <iostream>
 
auto main() -> int
{
    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

[edit] See also

(C++20)
finds the smallest integral power of two not less than the given value
(function template) [edit]
(C++20)
computes the result of bitwise right-rotation
(function template) [edit]
(C++20)
finds the smallest number of bits needed to represent the given value
(function template) [edit]
checks if a number is an integral power of two
(function template) [edit]