[intro.memory]

# 6 Basics [[basic]](./#basic)

## 6.8 Memory and objects [[basic.memobj]](basic.memobj#intro.memory)

### 6.8.1 Memory model [intro.memory]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/basic.tex#L3348)

The fundamental storage unit in the C++ memory model is the[*byte*](#def:byte "6.8.1 Memory model [intro.memory]")[.](#1.sentence-1)

A byte is at least large enough to contain
the ordinary literal encoding of any element of the basicliteral character set ([[lex.charset]](lex.charset "5.3.1 Character sets"))
and the eight-bit code units of the UnicodeUTF-8 encoding form
and is composed of a contiguous sequence of
bits,[19](#footnote-19 "The number of bits in a byte is reported by the macro CHAR_­BIT in the header <climits>.") the number of which is implementation-defined[.](#1.sentence-2)

The memory available to a C++ program consists of one or more sequences of
contiguous bytes[.](#1.sentence-3)

Every byte has a unique address[.](#1.sentence-4)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/basic.tex#L3370)

[*Note [1](#note-1)*:

The representation of types is described
in [[basic.types.general]](basic.types.general "6.9.1 General")[.](#2.sentence-1)

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/basic.tex#L3376)

A [*memory location*](#def:memory_location "6.8.1 Memory model [intro.memory]") 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[.](#3.sentence-1)

[*Note [2](#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[.](#3.sentence-2)

— *end note*]

Two or more [threads of
execution](intro.multithread#def:thread_of_execution "6.10.2 Multi-threaded executions and data races [intro.multithread]") can access separate memory
locations without interfering with each other[.](#3.sentence-3)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/basic.tex#L3391)

[*Note [3](#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[.](#4.sentence-1)

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[.](#4.sentence-2)

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[.](#4.sentence-3)

— *end note*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/basic.tex#L3403)

[*Example [1](#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[.](#5.sentence-1)

The bit-fieldsb and c together constitute the fourth memory location[.](#5.sentence-2)

The
bit-fields b and c cannot be concurrently modified, butb and a, for example, can be[.](#5.sentence-3)

— *end example*]

[19)](#footnote-19)[19)](#footnoteref-19)

The number of bits in a byte is reported by the macroCHAR_BIT in the header [<climits>](climits.syn#header:%3cclimits%3e "17.3.6 Header <climits> synopsis [climits.syn]")[.](#footnote-19.sentence-1)