std::bit_width
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bit_width (C++20) |
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Defined in header <bit>
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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, \(1 + \lfloor \log_2(x) \rfloor\) 1 + floor(log2(x)). If x
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
Parameters
| x | - | unsigned integer value |
Return value
Zero if x is zero; otherwise, one plus the base-2 logarithm of x
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
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made it int |
See also
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(C++20)
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counts the number of consecutive 0 bits, starting from the most significant bit (function template) |