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 | (1) | ||||||||
identifier (optional)attr (optional):sizebrace-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 |
[edit]Explanation
The type of a bit-field can only be integral (including bool) 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...7unsignedint b :3;}; int main(){ S s ={6}; ++s.b;// store the value 7 in the bit-fieldstd::cout<< s.b<<'\n'; ++s.b;// the value 8 does not fit in this bit-fieldstd::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 <bit>#include <cstdint>#include <iostream> struct S {// will usually occupy 2 bytes:unsignedchar b1 :3;// 1st 3 bits (in 1st byte) are b1unsignedchar:2;// next 2 bits (in 1st byte) are blocked out as unusedunsignedchar b2 :6;// 6 bits for b2 - doesn't fit into the 1st byte => starts a 2ndunsignedchar 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 Sauto 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-firststd::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: unusedunsignedchar b1 :3;unsignedchar:0;// start a new byteunsignedchar 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 in the range [0, 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;constint b =0; struct S {// simple casesint x1 :8=42;// OK; "= 42" is brace-or-equal-initializerint x2 :8{42};// OK; "{42}" is brace-or-equal-initializer // ambiguitiesint y1 :true?8: a =42;// OK; brace-or-equal-initializer is absentint y2 :true?8: b =42;// error: cannot assign to const intint y3 :(true?8: b)=42;// OK; "= 42" is brace-or-equal-initializerint z :1|| new int{0};// OK; brace-or-equal-initializer is absent}; | (since C++20) |
[edit]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++.
[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 |
|---|---|---|---|
| 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 |
[edit]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]
[edit]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 |
C documentation for Bit-fields | |
