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Difference between revisions of "cpp/numeric/bit width"

From cppreference.com
< cpp‎ | numeric
m ({{c}})
m (fmt.)
 
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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.
 
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===
 
===Parameters===
Line 32: Line 32:
 
int main()
 
int main()
 
{
 
{
     for (unsigned x{0}; x != 8; ++x)
+
     for (unsigned x{}; x != 010; ++x)
    {
+
         std::cout << "bit_width( "
         std::cout
+
                  << std::bitset<4>{x} << " ) = "
            << "bit_width( "
+
                  << std::bit_width(x) << '\n';
            << std::bitset<4>{x} << " ) = "
+
            << std::bit_width(x) << '\n';
+
    }
+
 
}
 
}
 
|output=
 
|output=
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===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{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 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}}
 
{{dr list end}}
  

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

#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

counts the number of consecutive 0 bits, starting from the most significant bit
(function template) [edit]