Memory model

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for(C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications(until C++17*)
noexcept specifier(C++11)
Exceptions
throw-expression try block	catch handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype(C++11) auto(C++11)	constexpr(C++11) consteval(C++20) constinit(C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization(C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr(C++11)
User-defined(C++11)
Utilities
Attributes(C++11)
Types
typedef declaration
Type alias declaration(C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier(C++11)   final specifier(C++11)	explicit(C++11) static
Special member functions
Default constructor Copy constructor Move constructor(C++11)	Copy assignment Move assignment(C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs(C++11)
Miscellaneous
Inline assembly	History of C++

 

Basic Concepts

Comments

ASCII

Punctuation

Names and identifiers

Types

Fundamental types

Objects

Scope

Object lifetime

Storage duration and linkage

Definitions and ODR

Name lookup

Qualified name lookup

Unqualified name lookup

The as-if rule

Undefined behavior

Memory model

Multi-threaded executions and data races(C++11)

Character sets and encodings

Phases of translation

The main function

Modules(C++20)

Contracts(C++26)

 

Defines the semantics of computer memory storage for the purpose of the C++ abstract machine.

The memory available to a C++ program is one or more contiguous sequences of bytes. Each byte in memory has a unique address.

Byte

A byte is the smallest addressable unit of memory. It is defined as a contiguous sequence of bits, large enough to hold

• the value of any UTF-8 code unit (256 distinct values) and of

any member of the basic execution character set.	(until C++23)
the ordinary literal encoding of any element of the basic literal character set.	(since C++23)

Similar to C, C++ supports bytes of sizes 8 bits and greater.

The typeschar, unsignedchar, and signedchar use one byte for both storage and value representation. The number of bits in a byte is accessible as CHAR_BIT or std::numeric_limits<unsignedchar>::digits.

Memory location

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 the largest contiguous sequence of bit-fields of non-zero length.

Note: 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.

struct S {char a;// memory location #1int b :5;// memory location #2int c :11, // memory location #2 (continued):0, d :8;// memory location #3struct{int ee :8;// memory location #4} e;} obj;// The object “obj” consists of 4 separate memory locations

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 1953	C++98	objects occupying the same storage were considered as different memory locations	memory location now refers to storage

See also

C documentation for Memory model