21.3. mutable Class Members

In Section 21.2, we introduced the const_cast operator, which allowed us to remove the “const-ness” of a type. A const_cast operation can also be applied to a data member of a const object from the body of a const member function of that object’s class. This enables the const member function to modify the data member, even though the object is considered to be const in the body of that function. Such an operation might be performed when most of an object’s data members should be considered const, but a particular data member still needs to be modified.

As an example, consider a linked list that maintains its contents in sorted order. Searching through the linked list does not require modifications to the data of the linked list, so the search function could be a const member function of the linked-list class. However, it’s conceivable that a linked-list object, in an effort to make future searches more efficient, might keep track of the location of the last successful match. If the next search operation attempts to locate an item that appears later in the list, the search could begin from the location of the last successful match, rather than from the beginning of the list. To do this, the const member function that performs the search must be able to modify the data member that keeps track of the last successful search.

If a data member such as the one described above should always be modifiable, C++ provides the storage-class specifier mutable as an alternative to const_cast. A mutable data member is always modifiable, even in a const member function or const object.

mutable and const_cast are used in different contexts. For a const object with no mutable data members, operator const_cast must be used every time a member is to be modified. This greatly reduces the chance of a member being accidentally modified because the member is not permanently modifiable. Operations involving const_cast are typically hidden in a member function’s implementation. The user of a class might not be aware that a member is being modified.