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 any member of the execution character set ([lex.charset]) and the eight-bit code units of the Unicode30 UTF-8 encoding form and is composed of a contiguous sequence of bits,31 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].

— 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 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]

30)

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.

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31)

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

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