bug1738.cpp

1     #include <iostream.h>
2     struct Art {
3         Art( const Art & ) { original = false; }
4         Art( ) { original = true; }
5         bool original;
6     };
7
8     struct Painting : public Art {
9         Painting( const Painting & arg ) :Art() { name = arg.name; }
10        Painting( const char *nm ) { name = nm; }
11        const char *name;
12        };
13    void print( const Painting &arg )
14        { cout << (arg.original ? "original" : "copy") << '\n' ; }
15
16    void print_copy( Painting arg ) { print( arg ); }
17
18    int main()
19        { Painting a("The Idiot"); print( a ); print_copy( a ); return 0; }

The programmer expected the first string printed to be "original" but was surprised when the second also printed "original". He expected argument passing to invoke a copy constructor that would result in a "copy". Where did the programmer go wrong?

bug1738.cpp lint Output

--- Module:   bug1738.cpp
                                            _
    Painting( const Painting & arg ) :Art() { name = arg.name; }
bug1738.cpp(9) : Info 1738: non copy constructor 'Art::Art(void)' used to
    initialize copy constructor
bug1738.cpp(9) : Warning 1554: Direct pointer copy of member 'Painting::name'
    within copy constructor: 'Painting::Painting(const Painting &)'
    _
    };
bug1738.cpp(12) : Info 1712: default constructor not defined for class
    'Painting'
                                                _
void print_copy( Painting arg ) { print( arg ); }
bug1738.cpp(16) : Info 1746: parameter 'arg' in function 'print_copy(Painting)'
    could be made const reference

Reference Manual Explanation

 
1738  non-copy constructor 'Symbol' used to initialize copy constructor
      -- In an initializer list for a copy constructor, a base class
      constructor was invoked. However, this base class constructor was
      not itself a copy constructor. We expect that copy constructors
      will invoke copy constructors. Was this an oversight or was there
      some good reason for choosing a different kind of constructor? If
      this was deliberate, suppress this message. See also message 1538.