7 Declarations [dcl.dcl]

7.1 Specifiers [dcl.spec]

7.1.6 Type specifiers [dcl.type] The cv-qualifiers [dcl.type.cv]

There are two cv-qualifiers, const and volatile. If a cv-qualifier appears in a decl-specifier-seq, the init-declarator-list of the declaration shall not be empty. [ Note: [basic.type.qualifier] and [dcl.fct] describe how cv-qualifiers affect object and function types.  — end note ] Redundant cv-qualifications are ignored. [ Note: For example, these could be introduced by typedefs. — end note ]

Note: Declaring a variable const can affect its linkage ([dcl.stc]) and its usability in constant expressions ([expr.const]). As described in [dcl.init], the definition of an object or subobject of const-qualified type must specify an initializer or be subject to default-initialization.  — end note ]

A pointer or reference to a cv-qualified type need not actually point or refer to a cv-qualified object, but it is treated as if it does; a const-qualified access path cannot be used to modify an object even if the object referenced is a non-const object and can be modified through some other access path. [ Note: Cv-qualifiers are supported by the type system so that they cannot be subverted without casting ([expr.const.cast]).  — end note ]

Except that any class member declared mutable ([dcl.stc]) can be modified, any attempt to modify a const object during its lifetime ([basic.life]) results in undefined behavior. [ Example:

const int ci = 3;               // cv-qualified (initialized as required)
ci = 4;                         // ill-formed: attempt to modify const

int i = 2;                      // not cv-qualified
const int* cip;                 // pointer to const int
cip = &i;                       // OK: cv-qualified access path to unqualified
*cip = 4;                       // ill-formed: attempt to modify through ptr to const

int* ip;
ip = const_cast<int*>(cip);     // cast needed to convert const int* to int*
*ip = 4;                        // defined: *ip points to i, a non-const object

const int* ciq = new const int (3);     // initialized as required
int* iq = const_cast<int*>(ciq);        // cast required
*iq = 4;                                // undefined: modifies a const object

For another example

struct X {
  mutable int i;
  int j;
struct Y {
  X x;

const Y y;
y.x.i++;                        // well-formed: mutable member can be modified
y.x.j++;                        // ill-formed: const-qualified member modified
Y* p = const_cast<Y*>(&y);      // cast away const-ness of y
p->x.i = 99;                    // well-formed: mutable member can be modified
p->x.j = 99;                    // undefined: modifies a const member

 — end example ]

If an attempt is made to refer to an object defined with a volatile-qualified type through the use of a glvalue with a non-volatile-qualified type, the program behavior is undefined.

Note: volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation. See [intro.execution] for detailed semantics. In general, the semantics of volatile are intended to be the same in C++ as they are in C.  — end note ]