# 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 bitfields and has special meaning.
brace-or-equal-initializer | - | default member initializer to be used with this bit-field

### 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 <iostream>
    #include <cstdint>
    #include <bit>

    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 14882:2023):

  - 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

bitset | implements constant length bit array (class template)
vector<bool> | space-efficient dynamic bitset (class template specialization)
Bit manipulation (C++20) | utilities to access, manipulate, and process individual bits and bit sequences

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