3.8 KiB
[intro.memory]
6 Basics [basic]
6.8 Memory and objects [basic.memobj]
6.8.1 Memory model [intro.memory]
The fundamental storage unit in the C++ memory model is thebyte.
A byte is at least large enough to contain the ordinary literal encoding of any element of the basicliteral character set ([lex.charset]) and the eight-bit code units of the UnicodeUTF-8 encoding form and is composed of a contiguous sequence of bits,19 the number of which is implementation-defined.
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].
â end note]
A memory location is the storage occupied by the object representation of 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.
â 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.
â end note]
[Example 1:
A class declared asstruct {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-fieldsd and e.ee are each separate memory locations, and can be modified concurrently without interfering with each other.
The bit-fieldsb and c together constitute the fourth memory location.
The bit-fields b and c cannot be concurrently modified, butb and a, for example, can be.
â end example]
The number of bits in a byte is reported by the macroCHAR_BIT in the header .