Difference between revisions of "cpp/numeric/bit width"
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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; |
}} | }} | ||
| − | If {{ | + | If {{c|x}} is not zero, calculates the number of bits needed to store the value {{c|x}}, that is, {{mathjax-or|1=\(1 + \lfloor \log_2(x) \rfloor\)|2=1 + floor(log{{su|b=2}}(x))}}. If {{c|x}} is zero, returns zero. |
| − | {{cpp/enable_if|{{tt|T}} is an unsigned integer type (that is, {{c|unsigned char}}, {{c|unsigned short}}, {{c|unsigned int}}, {{c|unsigned long}}, {{c|unsigned long long}}, or an extended unsigned integer type)}}. | + | {{cpp/enable_if|{{tt|T}} is an unsigned integer type (that is, {{c/core|unsigned char}}, {{c/core|unsigned short}}, {{c/core|unsigned int}}, {{c/core|unsigned long}}, {{c/core|unsigned long long}}, or an extended unsigned integer type)}}. |
| − | === | + | ===Parameters=== |
| − | + | {{par begin}} | |
| + | {{par|x|unsigned integer value}} | ||
| + | {{par end}} | ||
| − | === Notes === | + | ===Return value=== |
| + | Zero if {{c|x}} is zero; otherwise, one plus the base-2 logarithm of {{c|x}}, with any fractional part discarded. | ||
| + | |||
| + | ===Notes=== | ||
This function is equivalent to {{c|return std::numeric_limits<T>::digits - std::countl_zero(x);}}. | This function is equivalent to {{c|return std::numeric_limits<T>::digits - std::countl_zero(x);}}. | ||
| − | === Example === | + | {{feature test macro|__cpp_lib_int_pow2|std=C++20|value=202002L|[[cpp/numeric#Bit manipulation (since C++20)|Integral power-of-2 operations]]}} |
| − | {{example}} | + | |
| + | ===Example=== | ||
| + | {{example | ||
| + | |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 | ||
| + | }} | ||
| + | |||
| + | ===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/core|int}}}} | ||
| + | {{dr list end}} | ||
| − | === See also === | + | ===See also=== |
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/numeric/dsc countl_zero}} | + | {{dsc inc|cpp/numeric/dsc countl_zero}} |
{{dsc end}} | {{dsc end}} | ||
| − | {{langlinks|ja|zh}} | + | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} |
Latest revision as of 13:58, 2 November 2024
| 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 |
[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) |
counts the number of consecutive 0 bits, starting from the most significant bit (function template) |
