From Jason Turner

[class.bit]

C++14 → C++17 3.8 KB 48 lines
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Archived: C++14 | C++17
Markdown: C++14 | C++17
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  1. tmp/tmp_w6lyob5/{from.md → to.md} +28 -16
tmp/tmp_w6lyob5/{from.md → to.md} RENAMED
@@ -1,6 +1,6 @@
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- ## Bit-fields <a id="class.bit">[[class.bit]]</a>
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  A *member-declarator* of the form
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  specifies a bit-field; its length is set off from the bit-field name by
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  a colon. The optional *attribute-specifier-seq* appertains to the entity
@@ -12,49 +12,61 @@ the object representation ([[basic.types]]) of the bit-field’s type; in
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  such cases the extra bits are used as padding bits and do not
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  participate in the value representation ([[basic.types]]) of the
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  bit-field. Allocation of bit-fields within a class object is
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  *implementation-defined*. Alignment of bit-fields is
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  *implementation-defined*. Bit-fields are packed into some addressable
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- allocation unit. Bit-fields straddle allocation units on some machines
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- and not on others. Bit-fields are assigned right-to-left on some
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- machines, left-to-right on others.
 
 
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  A declaration for a bit-field that omits the *identifier* declares an
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- *unnamed* bit-field. Unnamed bit-fields are not members and cannot be
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- initialized. An unnamed bit-field is useful for padding to conform to
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- externally-imposed layouts. As a special case, an unnamed bit-field with
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- a width of zero specifies alignment of the next bit-field at an
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- allocation unit boundary. Only when declaring an unnamed bit-field may
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- the value of the *constant-expression* be equal to zero.
 
 
 
 
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  A bit-field shall not be a static member. A bit-field shall have
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  integral or enumeration type ([[basic.fundamental]]). A `bool` value
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  can successfully be stored in a bit-field of any nonzero size. The
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  address-of operator `&` shall not be applied to a bit-field, so there
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  are no pointers to bit-fields. A non-const reference shall not be bound
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- to a bit-field ([[dcl.init.ref]]). If the initializer for a reference
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- of type `const` `T&` is an lvalue that refers to a bit-field, the
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- reference is bound to a temporary initialized to hold the value of the
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- bit-field; the reference is not bound to the bit-field directly. See 
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- [[dcl.init.ref]].
 
 
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  If the value `true` or `false` is stored into a bit-field of type `bool`
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  of any size (including a one bit bit-field), the original `bool` value
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  and the value of the bit-field shall compare equal. If the value of an
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  enumerator is stored into a bit-field of the same enumeration type and
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  the number of bits in the bit-field is large enough to hold all the
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  values of that enumeration type ([[dcl.enum]]), the original enumerator
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  value and the value of the bit-field shall compare equal.
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  ``` cpp
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  enum BOOL { FALSE=0, TRUE=1 };
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  struct A {
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  BOOL b:1;
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  };
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  A a;
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  void f() {
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  a.b = TRUE;
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  if (a.b == TRUE) // yields true
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- { /* ... */ }
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  }
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  ```
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+ ### Bit-fields <a id="class.bit">[[class.bit]]</a>
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  A *member-declarator* of the form
4
 
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  specifies a bit-field; its length is set off from the bit-field name by
6
  a colon. The optional *attribute-specifier-seq* appertains to the entity
 
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  such cases the extra bits are used as padding bits and do not
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  participate in the value representation ([[basic.types]]) of the
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  bit-field. Allocation of bit-fields within a class object is
15
  *implementation-defined*. Alignment of bit-fields is
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  *implementation-defined*. Bit-fields are packed into some addressable
17
+ allocation unit.
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+
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+ [*Note 1*: Bit-fields straddle allocation units on some machines and
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+ not on others. Bit-fields are assigned right-to-left on some machines,
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+ left-to-right on others. — *end note*]
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  A declaration for a bit-field that omits the *identifier* declares an
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+ *unnamed bit-field*. Unnamed bit-fields are not members and cannot be
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+ initialized.
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+
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+ [*Note 2*: An unnamed bit-field is useful for padding to conform to
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+ externally-imposed layouts. — *end note*]
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+
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+ As a special case, an unnamed bit-field with a width of zero specifies
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+ alignment of the next bit-field at an allocation unit boundary. Only
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+ when declaring an unnamed bit-field may the value of the
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+ *constant-expression* be equal to zero.
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  A bit-field shall not be a static member. A bit-field shall have
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  integral or enumeration type ([[basic.fundamental]]). A `bool` value
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  can successfully be stored in a bit-field of any nonzero size. The
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  address-of operator `&` shall not be applied to a bit-field, so there
39
  are no pointers to bit-fields. A non-const reference shall not be bound
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+ to a bit-field ([[dcl.init.ref]]).
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+
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+ [*Note 3*: If the initializer for a reference of type `const` `T&` is
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+ an lvalue that refers to a bit-field, the reference is bound to a
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+ temporary initialized to hold the value of the bit-field; the reference
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+ is not bound to the bit-field directly. See 
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+ [[dcl.init.ref]]. — *end note*]
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  If the value `true` or `false` is stored into a bit-field of type `bool`
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  of any size (including a one bit bit-field), the original `bool` value
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  and the value of the bit-field shall compare equal. If the value of an
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  enumerator is stored into a bit-field of the same enumeration type and
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  the number of bits in the bit-field is large enough to hold all the
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  values of that enumeration type ([[dcl.enum]]), the original enumerator
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  value and the value of the bit-field shall compare equal.
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+ [*Example 1*:
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+
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  ``` cpp
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  enum BOOL { FALSE=0, TRUE=1 };
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  struct A {
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  BOOL b:1;
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  };
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  A a;
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  void f() {
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  a.b = TRUE;
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  if (a.b == TRUE) // yields true
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+ { ... }
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  }
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  ```
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+ — *end example*]
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+