6 Basic concepts [basic]

6.9 Types [basic.types]

6.9.3 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 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.

The cv-qualified or cv-unqualified versions of a type are distinct types; however, they shall have the same representation and alignment requirements.55

A compound type 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 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. 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.