6 Basics [basic]

6.8 Types [basic.types]

6.8.3 CV-qualifiers [basic.type.qualifier]

A type mentioned in [basic.fundamental] and [basic.compound] is a cv-unqualified type.

Each type which is a cv-unqualified 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.

(1.1)
A const object is an object of type const T or a non-mutable subobject of a const object.
(1.2)
A volatile object is an object of type volatile T or a subobject of a volatile object.
(1.3)
A const volatile object is an object of type const volatile T, a non-mutable subobject of a const volatile 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]).43

Except for array types, a compound type ([basic.compound]) is not cv-qualified by the cv-qualifiers (if any) of the types from which it is compounded.

An array type whose elements are cv-qualified is also considered to have the same cv-qualifications as its elements.

[Note

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 ([dcl.array]).

— end note

]

[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

]

[Note

See [dcl.fct] and [class.this] regarding function types that have cv-qualifiers.

— end note

]

There is a partial ordering on cv-qualifiers, so that a type can be said to be more cv-qualified than another.

Table 13 shows the relations that constitute this ordering.

Table 13: Relations on const and volatile [tab:basic.type.qualifier.rel]

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

]

43)

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.

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