[ Note: Declaring a variable const can affect its linkage ([dcl.stc]) and its usability in constant expressions. 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. — end note ]
Except that any class member declared mutable can be modified, any attempt to modify a const object during its lifetime results in undefined behavior. [ Example:
const int ci = 3; ci = 4; int i = 2; const int* cip; cip = &i; *cip = 4; int* ip; ip = const_cast<int*>(cip); *ip = 4; const int* ciq = new const int (3); int* iq = const_cast<int*>(ciq); *iq = 4;
For another example,
struct X {
mutable int i;
int j;
};
struct Y {
X x;
Y();
};
const Y y;
y.x.i++; y.x.j++; Y* p = const_cast<Y*>(&y); p->x.i = 99; p->x.j = 99;
— end example ]
The semantics of an access through a volatile glvalue are implementation-defined. If an attempt is made to access an object defined with a volatile-qualified type through the use of a non-volatile glvalue, the 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. Furthermore, for some implementations, volatile might indicate that special hardware instructions are required to access the object. 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 ]
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