C++ Language/Objects/ConstMembers/ConstMemberFunctions

A member function automatically has access to a  pointer variable whose type is   and which is automatically pointing at the   object being manipulated by this member function. But you could have defined the member function as, in which case the type of   becomes   (that function's body can view data members but not alter them).

This restriction can be ignored for individual data members (on a case-by-case basis) but marking the data member with.

Additional information about const member functions