6 Basics [basic]

6.8 Types [basic.types]

6.8.5 CV-qualifiers [basic.type.qualifier]

Each type other than a function or reference type is part of a group of four distinct, but related, types: a cv-unqualified version, a const-qualified version, a volatile-qualified version, and a const-volatile-qualified version.

The types in each such group shall have the same representation and alignment requirements ([basic.align]).34

A function or reference type is always cv-unqualified.

(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.

[Note 1:

The type of an object ([intro.object]) includes the cv-qualifiers specified in the decl-specifier-seq ([dcl.spec]), declarator ([dcl.decl]), type-id ([dcl.name]), or new-type-id ([expr.new]) when the object is created.

— end note]

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 2:

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 1: 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 3:

See [dcl.fct] and [over.match.funcs] 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 16 shows the relations that constitute this ordering.

Table 16: 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 2:

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]

34)34)

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.