5 Expressions [expr]

The result of the expression const_cast<T>(v) is of type T. If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) standard conversions are performed on the expression v. Conversions that can be performed explicitly using const_cast are listed below. No other conversion shall be performed explicitly using const_cast.

[ Note: Subject to the restrictions in this section, an expression may be cast to its own type using a const_cast operator.  — end note ]

For two pointer types T1 and T2 where

and

where T is any object type or the void type and where cv_1,k and cv_2,k may be different cv-qualifications, a prvalue of type T1 may be explicitly converted to the type T2 using a const_cast. The result of a pointer const_cast refers to the original object.

For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:

• (4.1)

  an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;

• (4.2)

  a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and

• (4.3)

  if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>.

The result of a reference const_cast refers to the original object.

For a const_cast involving pointers to data members, multi-level pointers to data members and multi-level mixed pointers and pointers to data members ([conv.qual]), the rules for const_cast are the same as those used for pointers; the “member” aspect of a pointer to member is ignored when determining where the cv-qualifiers are added or removed by the const_cast. The result of a pointer to data member const_cast refers to the same member as the original (uncast) pointer to data member.

A null pointer value ([conv.ptr]) is converted to the null pointer value of the destination type. The null member pointer value ([conv.mem]) is converted to the null member pointer value of the destination type.

[ Note: Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier74 may produce undefined behavior ([dcl.type.cv]).  — end note ]

The following rules define the process known as casting away constness. In these rules Tn and Xn represent types. For two pointer types:

casting from X1 to X2 casts away constness if, for a non-pointer type T there does not exist an implicit conversion (Clause [conv]) from:

to

Casting from an lvalue of type T1 to an lvalue of type T2 using an lvalue reference cast or casting from an expression of type T1 to an xvalue of type T2 using an rvalue reference cast casts away constness if a cast from a prvalue of type “pointer to T1” to the type “pointer to T2” casts away constness.

Casting from a prvalue of type “pointer to data member of X of type T1” to the type “pointer to data member of Y of type T2” casts away constness if a cast from a prvalue of type “pointer to T1” to the type “pointer to T2” casts away constness.

For multi-level pointer to members and multi-level mixed pointers and pointer to members ([conv.qual]), the “member” aspect of a pointer to member level is ignored when determining if a const cv-qualifier has been cast away.

[ Note: some conversions which involve only changes in cv-qualification cannot be done using const_cast. For instance, conversions between pointers to functions are not covered because such conversions lead to values whose use causes undefined behavior. For the same reasons, conversions between pointers to member functions, and in particular, the conversion from a pointer to a const member function to a pointer to a non-const member function, are not covered.  — end note ]