12 Classes [class]

12.2 Class members [class.mem]

12.2.4 Bit-fields [class.bit]

1
#
A member-declarator of the form
identifier attribute-specifier-seq : constant-expression
specifies a bit-field; its length is set off from the bit-field name by a colon.
The optional attribute-specifier-seq appertains to the entity being declared.
The bit-field attribute is not part of the type of the class member.
The constant-expression shall be an integral constant expression with a value greater than or equal to zero.
The value of the integral constant expression may be larger than the number of bits in the object representation ([basic.types]) of the bit-field's type; in such cases the extra bits are used as padding bits and do not participate in the value representation ([basic.types]) of the bit-field.
Allocation of bit-fields within a class object is implementation-defined.
Alignment of bit-fields is implementation-defined.
Bit-fields are packed into some addressable allocation unit.
[ Note
:
Bit-fields straddle allocation units on some machines and not on others.
Bit-fields are assigned right-to-left on some machines, left-to-right on others.
— end note
 ]
2
#
A declaration for a bit-field that omits the identifier declares an unnamed bit-field.
Unnamed bit-fields are not members and cannot be initialized.
[ Note
:
An unnamed bit-field is useful for padding to conform to externally-imposed layouts.
— end note
 ]
As a special case, an unnamed bit-field with a width of zero specifies alignment of the next bit-field at an allocation unit boundary.
Only when declaring an unnamed bit-field may the value of the constant-expression be equal to zero.
3
#
A bit-field shall not be a static member.
A bit-field shall have integral or enumeration type ([basic.fundamental]).
A bool value can successfully be stored in a bit-field of any nonzero size.
The address-of operator & shall not be applied to a bit-field, so there are no pointers to bit-fields.
A non-const reference shall not be bound to a bit-field ([dcl.init.ref]).
[ Note
:
If the initializer for a reference of type const T& is an lvalue that refers to a bit-field, the reference is bound to a temporary initialized to hold the value of the bit-field; the reference is not bound to the bit-field directly.
— end note
 ]
4
#
If the value true or false is stored into a bit-field of type bool of any size (including a one bit bit-field), the original bool value and the value of the bit-field shall compare equal.
If the value of an enumerator is stored into a bit-field of the same enumeration type and the number of bits in the bit-field is large enough to hold all the values of that enumeration type ([dcl.enum]), the original enumerator value and the value of the bit-field shall compare equal.
[ Example
: 
enum BOOL { FALSE=0, TRUE=1 };
struct A {
  BOOL b:1;
};
A a;
void f() {
  a.b = TRUE;
  if (a.b == TRUE)              // yields true
    { /* ... */ }
}
— end example
 ]