Section references in this document reflect the section numbering of document WG21 N4659.

2253. Unnamed bit-fields and zero-initialization

The current wording of the Standard does not clearly state that zero-initialization applies to unnamed bit-fields.

Notes from the December, 2016 teleconference:

The consensus was that unnamed bit-fields do constitute padding; more generally, padding should be normatively defined, along the lines suggested in 12.2.4 [class.bit] paragraphs 1-2.

Proposed resolution (March, 2017):

1. Change 6.9 [basic.types] paragraph 4 as follows:

The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T). The value representation of an object is the set of bits that hold the value of type T. Bits in the object representation that are not part of the value representation are padding bits. For trivially copyable types, the value representation is a set of bits in the object representation that determines a value, which is one discrete element of an implementation-defined set of values.⁴⁴

2. Change 11.6 [dcl.init] paragraph 6 as follows:

To zero-initialize an object or reference of type T means:

• if T is a scalar type (6.9 [basic.types]), the object is initialized to the value obtained by converting the integer literal 0 (zero) to T;¹⁰³

• if T is a (possibly cv-qualified) non-union class type, its padding bits are initialized to zero bits and each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;

• if T is a (possibly cv-qualified) union type, its padding bits are initialized to zero bits and the object's first non-static named data member is zero-initialized and padding is initialized to zero bits;

• ...

3. Change 12.2.4 [class.bit] paragraph 1 as follows:

...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 (6.9 [basic.types]) of the bit-field's type; in such cases the extra bits are used as padding bits (6.9 [basic.types])and do not participate in the value representation (6.9 [basic.types]) of the bit-field. Allocation of bit-fields...

4. Change 6.9.1 [basic.fundamental] paragraph 1 as follows:

...For narrow character types, all bits of the object representation participate in the value representation. [Note: A bit-field of narrow character type whose length is larger than the number of bits in the object representation of that type has padding bits; see 12.2.4 [class.bit] 6.9 [basic.types]. —end note] For unsigned narrow character types...