std:: bit_width
|
Defined in header
<bit>
|
||
|
template
<
class
T
>
constexpr int bit_width ( T x ) noexcept ; |
(since C++20) | |
If x is not zero, calculates the number of bits needed to store the value x , that is, 1 + floor(log 2 (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, one plus the base-2 logarithm of x , with any fractional part discarded.
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 |
Example
#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
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 |
See also
|
(C++20)
|
counts the number of consecutive
0
bits, starting from the most significant bit
(function template) |