6 Basic concepts [basic]

6.7 Types [basic.types]

6.7.3 CV-qualifiers [basic.type.qualifier]

Each type which is a cv-unqualified complete or incomplete object type or is void ([basic.types]) has three corresponding cv-qualified versions of its type: a const-qualified version, a volatile-qualified version, and a const-volatile-qualified version.
The type of an object ([intro.object]) includes the cv-qualifiers specified in the decl-specifier-seq, declarator, type-id, or new-type-id when the object is created.
  • A const object is an object of type const T or a non-mutable subobject of such an object.
  • A volatile object is an object of type volatile T, a subobject of such an object, or a mutable subobject of a const volatile object.
  • A const volatile object is an object of type const volatile T, a non-mutable subobject of such an object, a const subobject of a volatile object, or a non-mutable volatile subobject of a const object.
The cv-qualified or cv-unqualified versions of a type are distinct types; however, they shall have the same representation and alignment requirements ([basic.align]).51
A compound type ([basic.compound]) is not cv-qualified by the cv-qualifiers (if any) of the types from which it is compounded.
Any cv-qualifiers applied to an array type affect the array element type ([dcl.array]).
See [dcl.fct] and [class.this] regarding function types that have cv-qualifiers.
There is a partial ordering on cv-qualifiers, so that a type can be said to be more cv-qualified than another.
Table 10 shows the relations that constitute this ordering.
Table 10 — Relations on const and volatile
no cv-qualifier
<
const
no cv-qualifier
<
volatile
no cv-qualifier
<
const volatile
const
<
const volatile
volatile
<
const volatile
In this document, the notation cv (or cv1, cv2, etc.)
, used in the description of types, represents an arbitrary set of cv-qualifiers, i.e., one of {const}, {volatile}, {const, volatile}, or the empty set.
For a type cv T, the top-level cv-qualifiers of that type are those denoted by cv.
[Example
:
The type corresponding to the type-id const int& has no top-level cv-qualifiers.
The type corresponding to the type-id volatile int * const has the top-level cv-qualifier const.
For a class type C, the type corresponding to the type-id void (C​::​* volatile)(int) const has the top-level cv-qualifier volatile.
end example
]
Cv-qualifiers applied to an array type attach to the underlying element type, so the notation “cv T”, where T is an array type, refers to an array whose elements are so-qualified.
An array type whose elements are cv-qualified is also considered to have the same cv-qualifications as its elements.
[Example
:
typedef char CA[5];
typedef const char CC;
CC arr1[5] = { 0 };
const CA arr2 = { 0 };
The type of both arr1 and arr2 is “array of 5 const char”, and the array type is considered to be const-qualified.
end example
]
The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and non-static data members of unions.