3 Basic concepts [basic]

3.9 Types [basic.types]

3.9.3 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 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 term object type ([intro.object]) includes the cv-qualifiers specified in the decl-specifier-seq ([dcl.spec]), declarator (Clause [dcl.decl]), type-id ([dcl.name]), or new-type-id ([expr.new]) 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]).53

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, not the array 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 [tab:relations.on.const.and.volatile] shows the relations that constitute this ordering.

Table 9 — 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 International Standard, 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. 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.