Bit-field - cppreference.com (original) (raw)

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

[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   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:

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

#include #include #include   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:

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   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:

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; 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 };

[edit] Notes

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

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 valueof an assignment to a bit-field is a bit-field added bit-field specifications foroperators which may return lvalues
CWG 739 C++98 signedness of bit-fields that are neither declaredsigned 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-qualifiedenumeration types

[edit] References

[edit] See also