bug1763.cpp

1    #include <stdio.h>
2
3    class Drink
4        {
5        public:
6        Drink( char *name_ ) : name( name_ ) {}
7        char *name;
8        char & get(int i) const { return name[i]; }
9        };
10
11   void stir( char &ch ) { ++ch; }
12
13   int main()
14       {
15       Drink Martini("Martini");
16       stir( Martini.get(2) );
17       printf( "%s\n", Martini.name );
18       return 0;
19       }    

At our New Year's Eve party, Letter Man, accessing our Drink through a const function, turned our "Martini" into a "Mastini". How was he able to get away with this?

bug1763.cpp lint Output

--- Module:   bug1763.cpp
                                              _
    char & get(int i) const { return name[i]; }
bug1763.cpp(8) : Info 1763: Member function 'Drink::get(int) const' marked as
    const indirectly modifies class
    _
    };
bug1763.cpp(9) : Info 1712: default constructor not defined for class 'Drink'
                           _
    Drink Martini("Martini");
bug1763.cpp(15) : Info 1776: Converting a string literal to char * is not const
    safe (arg. no. 1)  

Reference Manual Explanation

1763  Member function 'Symbol' marked as const indirectly modifies class  -- The
      designated symbol is a member function declared as const.  Though technically
      valid, the const may be misleading because the member function modifies
      (or exposes) information indirectly referenced by the object.  For example:

                class X
                   {
                   char *pc;
                   char & get(int i) const { return pc[i]; }
                   };

      results in Info 1763 for function X::get.  This is because the function
      exposes information indirectly held by the class X.

      Experts [24] recommend that a pair of functions be made available in
      this situation:

                class X
                   {
                   char *pc;
                   const char & get(int i) const { return pc[i]; }
                   char & get(int i) { return pc[i]; }
                   };

      In this way, if the object is const then only the const function will
      be called, which will return the protected reference.  Related messages
      are also  1762 and  1962.