Memory model
inline specifier noexcept specifier (C++11)typedef declaration 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.
[edit] 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-8code unit (256 distinct values) and of
- any member of the basic execution character set.
- the ordinary literal encoding of any element of the basic literal character set.
Similar to C, C++ supports bytes of sizes 8 bits and greater.
The types char, unsigned char, and signed char 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 <unsigned char>::digits.
[edit] 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 #1 int b : 5; // memory location #2 int c : 11, // memory location #2 (continued) : 0, d : 8; // memory location #3 struct { int ee : 8; // memory location #4 } e; } obj; // The object "obj" consists of 4 separate memory locations
[edit] 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 |