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Bit-field

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Declares a class data member with explicit size, in bits. Adjacent bit-field members may (or may not) be packed to share and straddle the individual bytes.

A bit-field declaration is a class data member declaration which uses the following declarator:

identifier (optional) attr (optional) : size (1)
identifier (optional) attr (optional) : size brace-or-equal-initializer (2) (since C++20)

The type of the bit-field is introduced by the decl-specifier-seq of the declaration syntax.

attr - (since C++11) sequence of any number of attributes
identifier - the name of the bit-field that is being declared. The name is optional: unnamed bit-fields introduce the specified number of padding bits.
size - an integral constant expression with a value greater or equal to zero. When greater than zero, this is the number of bits that this bit-field will occupy. The value zero is only allowed for nameless bit-fields and has special meaning.
brace-or-equal-initializer - default member initializer to be used with this bit-field

Contents

Explanation

The type of a bit-field can only be integral or (possibly cv-qualified) enumeration type, an unnamed bit-field cannot be declared with a cv-qualified type.

A bit-field cannot be a static data member.

There are no bit-field prvalues: lvalue-to-rvalue conversion always produces an object of the underlying type of the bit-field.

The number of bits in a bit-field sets the limit to the range of values it can hold:

Run this code
#include <iostream>
 
struct S
{
    // three-bit unsigned field, allowed values are 0...7
    unsigned int b : 3;
};
 
int main()
{
    S s = {6};
 
    ++s.b; // store the value 7 in the bit-field
    std::cout << s.b << '\n';
 
    ++s.b; // the value 8 does not fit in this bit-field
    std::cout << s.b << '\n'; // formally implementation-defined, typically 0
}

Possible output: 

7
0

Multiple adjacent bit-fields are usually packed together (although this behavior is implementation-defined):

Run this code
#include <bit>
#include <cstdint>
#include <iostream>
 
struct S
{
    // will usually occupy 2 bytes:
    unsigned char b1 : 3; // 1st 3 bits (in 1st byte) are b1
    unsigned char    : 2; // next 2 bits (in 1st byte) are blocked out as unused
    unsigned char b2 : 6; // 6 bits for b2 - doesn't fit into the 1st byte => starts a 2nd
    unsigned char b3 : 2; // 2 bits for b3 - next (and final) bits in the 2nd byte
};
 
int main()
{
    std::cout << sizeof(S) << '\n'; // usually prints 2
 
    S s;
    // set distinguishable field values
    s.b1 = 0b111;
    s.b2 = 0b101111;
    s.b3 = 0b11;
 
    // show layout of fields in S
    auto i = std::bit_cast<std::uint16_t>(s);
    // usually prints 1110000011110111
    // breakdown is:  \_/\/\_/\____/\/
    //                 b1 u a   b2  b3
    // where "u" marks the unused :2 specified in the struct, and
    // "a" marks compiler-added padding to byte-align the next field.
    // Byte-alignment is happening because b2's type is declared unsigned char;
    // if b2 were declared uint16_t there would be no "a", b2 would abut "u".
    for (auto b = i; b; b >>= 1) // print LSB-first
        std::cout << (b & 1);
    std::cout << '\n';
}

Possible output: 

2
1110000011110111

The special unnamed bit-field of size zero can be forced to break up padding. It specifies that the next bit-field begins at the beginning of its allocation unit:

Run this code
#include <iostream>
 
struct S
{
    // will usually occupy 2 bytes:
    // 3 bits: value of b1
    // 5 bits: unused
    // 2 bits: value of b2
    // 6 bits: unused
    unsigned char b1 : 3;
    unsigned char :0; // start a new byte
    unsigned char b2 : 2;
};
 
int main()
{
    std::cout << sizeof(S) << '\n'; // usually prints 2
                                    // would usually print 1 if not for
                                    // the padding break in line 11
}

Possible output: 

2

If the specified size of the bit-field is greater than the size of its type, the value is limited by the type: a std::uint8_t b : 1000; would still hold values between 0 and 255. the extra bits are padding bits.

Because bit-fields do not necessarily begin at the beginning of a byte, address of a bit-field cannot be taken. Pointers and non-const references to bit-fields are not possible. When initializing a const reference from a bit-field, a temporary is created (its type is the type of the bit-field), copy initialized with the value of the bit-field, and the reference is bound to that temporary.

There are no default member initializers for bit-fields: int b : 1 = 0; and int b : 1 {0} are ill-formed.

(until C++20)

In case of ambiguity between the size of the bit-field and the default member initializer, the longest sequence of tokens that forms a valid size is chosen: 

int a;
const int b = 0;
 
struct S
{
    // simple cases
    int x1 : 8 = 42;              // OK; "= 42" is brace-or-equal-initializer
    int x2 : 8 {42};              // OK; "{42}" is brace-or-equal-initializer
 
    // ambiguities
    int y1 : true ? 8 : a = 42;   // OK; brace-or-equal-initializer is absent
    int y2 : true ? 8 : b = 42;   // error: cannot assign to const int
    int y3 : (true ? 8 : b) = 42; // OK; "= 42" is brace-or-equal-initializer
    int z : 1 || new int{0};      // OK; brace-or-equal-initializer is absent
};
 (since C++20)

Notes

The following properties of bit-fields are implementation-defined:

  • The value that results from assigning or initializing a signed bit-field with a value out of range, or from incrementing a signed bit-field past its range.
  • Everything about the actual allocation details of bit-fields within the class object.
  • For example, on some platforms, bit-fields don't straddle bytes, on others they do.
  • Also, on some platforms, bit-fields are packed left-to-right, on others right-to-left.

In the C programming language, the width of a bit-field cannot exceed the width of the underlying type, and whether int bit-fields that are not explicitly signed or unsigned are signed or unsigned is implementation-defined. For example, int b:3; may have the range of values 0..7 or -4..3 in C, but only the latter choice is allowed in C++.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 324 C++98 it was unspecified whether the return value
of an assignment to a bit-field is a bit-field 		added bit-field specifications for
operators which may return lvalues
CWG 739 C++98 signedness of bit-fields that are neither declared
signed nor unsigned were implementation-defined 		consistent with underlying types
CWG 2229 C++98 unnamed bit-fields could be declared with a cv-qualified type prohibited
CWG 2511 C++98 cv-qualifications were not allowed in bit-field types bit-fields can have cv-qualified
enumeration types

References

  • C++23 standard (ISO/IEC 14882:2024):
  • 11.4.10 Bit-fields [class.bit]
  • C++20 standard (ISO/IEC 14882:2020):
  • 11.4.9 Bit-fields [class.bit]
  • C++17 standard (ISO/IEC 14882:2017):
  • 12.2.4 Bit-fields [class.bit]
  • C++14 standard (ISO/IEC 14882:2014):
  • 9.6 Bit-fields [class.bit]
  • C++11 standard (ISO/IEC 14882:2011):
  • 9.6 Bit-fields [class.bit]
  • C++03 standard (ISO/IEC 14882:2003):
  • 9.6 Bit-fields [class.bit]
  • C++98 standard (ISO/IEC 14882:1998):
  • 9.6 Bit-fields [class.bit]

See also

 implements constant length bit array
(class template) [edit]
space-efficient dynamic bitset
(class template specialization) [edit]
Bit manipulation (C++20) utilities to access, manipulate, and process individual bits and bit sequences
C documentation for Bit-fields
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