std::bit_width
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bit_width
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Defined in header
<bit>
template< class T >
constexpr int bit_width( T x ) noexcept;
(since C++20)
constexpr int bit_width( T x ) noexcept;
If x is not zero, calculates the number of bits needed to store the value x, that is, \(1 + \lfloor \log_2(x) \rfloor\)1 + floor(log2(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).
[edit] Parameters
x
-
unsigned integer value
[edit] Return value
Zero if x is zero; otherwise, one plus the base-2 logarithm of x, with any fractional part discarded.
[edit] Notes
This function is equivalent to return std::numeric_limits <T>::digits - std::countl_zero (x);.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_int_pow2 |
202002L |
(C++20) | Integral power-of-2 operations |
[edit] Example
Run this code
#include <bit> #include <bitset> #include <iostream> int main() { for (unsigned x{}; x != 010; ++x) std::cout << "bit_width( " << std::bitset <4>{x} << " ) = " << std::bit_width(x) << '\n'; }
Output:
bit_width( 0000 ) = 0 bit_width( 0001 ) = 1 bit_width( 0010 ) = 2 bit_width( 0011 ) = 2 bit_width( 0100 ) = 3 bit_width( 0101 ) = 3 bit_width( 0110 ) = 3 bit_width( 0111 ) = 3
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3656 | C++20 | the return type of bit_width is the same as the type of its function argument
|
made it int |
[edit] See also
(C++20)
(function template) [edit]