bug1772.cpp

1    #include <stdio.h>
2    class R
3        {
4        public:
5        int k, depth;
6        R() :k(0), depth(0) {}
7        const R&  operator=( const R & r )
8            {
9            if( this != &r )
10               { k = r.k; depth = r.depth+1; }
11           return r;
12           }
13       };
14   int main()
15       {
16       R r1, r2, r3;
17       r1 = r2 = r3;
18       printf( "%d %d %d\n",
19               r1.depth, r2.depth, r3.depth );
20       }

Wacknia is attempting to simulate the recording industry where a copy of a recording R is less faithful than the original. Member depth is meant to record the degree of imperfection. But to her surprise the program above is printing "1 1 0" rather than the expected "2 1 0". What's going on?

bug1772.cpp lint Output

--- Module:   bug1772.cpp (C++)
                _
        return r;
bug1772.cpp(11) : Info 1772: Assignment operator 'R::operator=(const R &)' is
    not returning *this

Reference Manual Explanation

1772   Assignment operator 'Symbol' is not returning *this  -- The assignment operator
       should return *this.  This is to allow for multiple assignments as in:

             a = b = c;

       It is also better to return the object that has just been modified rather than the argument.