# std::bit_floor

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< 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 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

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<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{}; 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)

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