Difference between revisions of "cpp/numeric/bit width"
From cppreference.com
m (→Notes: + feature test macro.) |
m (LWG3656 applied) |
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{{ddcl|since=c++20|header=bit| | {{ddcl|since=c++20|header=bit| | ||
template< class T > | template< class T > | ||
| − | constexpr | + | constexpr int bit_width( T x ) noexcept; |
}} | }} | ||
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bit_width( 0111 ) = 3 | bit_width( 0111 ) = 3 | ||
}} | }} | ||
| + | |||
| + | ===Defect reports=== | ||
| + | {{dr list begin}} | ||
| + | {{dr list item|wg=lwg|dr=3656|std=C++20|before=the return type of {{tt|bit_width}} is the same as the type of its function argument |after=made it {{c|int}}}} | ||
| + | {{dr list end}} | ||
=== See also === | === See also === | ||
Revision as of 19:06, 27 July 2022
| 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(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).
Contents |
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 |
Example
Run this code
#include <bit> #include <bitset> #include <iostream> int main() { for (unsigned x{0}; x != 8; ++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) |
