Bit-field
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
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(1) | ||||||||
identifier(optional) attr(optional) : size brace-or-equal-initializer
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(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:
#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):
#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:
#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.
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There are no default member initializers for bit-fields: int b : 1 = 0; and int b : 1 {0} are ill-formed. |
(until C++20) |
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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
| implements constant length bit array (class template) | |
| space-efficient dynamic bitset (class template specialization) | |
| Bit manipulation (C++20) | utilities to access, manipulate, and process individual bits and bit sequences |