6 Basics [basic]

6.7 Memory and objects [basic.memobj]

6.7.1 Memory model [intro.memory]

The fundamental storage unit in the C++ memory model is the byte.

A byte is at least large enough to contain the ordinary literal encoding of any element of the basic literal character set ([lex.charset]) and the eight-bit code units of the Unicode19 UTF-8 encoding form and is composed of a contiguous sequence of bits,20 the number of which is implementation-defined.

The least significant bit is called the low-order bit; the most significant bit is called the high-order bit.

The memory available to a C++ program consists of one or more sequences of contiguous bytes.

Every byte has a unique address.

[Note 1:

The representation of types is described in [basic.types.general].

— end note]

A memory location is either an object of scalar type that is not a bit-field or a maximal sequence of adjacent bit-fields all having nonzero width.

[Note 2:

Various features of the language, such as references and virtual functions, might involve additional memory locations that are not accessible to programs but are managed by the implementation.

— end note]

Two or more threads of execution ([intro.multithread]) can access separate memory locations without interfering with each other.

[Note 3:

Thus a bit-field and an adjacent non-bit-field are in separate memory locations, and therefore can be concurrently updated by two threads of execution without interference.

The same applies to two bit-fields, if one is declared inside a nested struct declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field declaration.

It is not safe to concurrently update two bit-fields in the same struct if all fields between them are also bit-fields of nonzero width.

— end note]

[Example 1:

A class declared as struct { char a; int b:5, c:11, :0, d:8; struct {int ee:8;} e; }; contains four separate memory locations: The member a and bit-fields d and e.ee are each separate memory locations, and can be modified concurrently without interfering with each other.

The bit-fields b and c together constitute the fourth memory location.

The bit-fields b and c cannot be concurrently modified, but b and a, for example, can be.

— end example]

19)19)

Unicode® is a registered trademark of Unicode, Inc.

This information is given for the convenience of users of this document and does not constitute an endorsement by ISO or IEC of this product.

20)20)

The number of bits in a byte is reported by the macro CHAR_BIT in the header <climits>.