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From: sdm@cs.brown.edu (Scott Meyers)
Newsgroups: comp.std.c++
Subject: Virtual Functions and Default Parameter Values
Message-ID: <79633@brunix.UUCP>
Date: 28 Jun 91 15:47:17 GMT
Sender: news@brunix.UUCP
Reply-To: sdm@cs.brown.edu (Scott Meyers)
Organization: Brown University Department of Computer Science
Lines: 43

Here's an interesting anomaly: a virtual function taking a default
parameter value can be redefined in a derived class with a different
default value.  This can lead to unexpected behavior:

  #include <iostream.h>

  class Base { 
  public:
    virtual void f(char * className = "Base") { cout << className << endl; }
  };

  class Derived: public Base {
  public:
    virtual void f(char * className = "Derived") { cout << className << endl; }
  };

  main()
  {
    Derived *d = new Derived;
    d->f();                  // prints "Derived"

    Base *b = d;
    b->f();                  // prints "Base"
  }

Both calls to f resolve to Derived::f, as expected, but the parameter has
different values, depending on which pointer was used to invoke it.

As I understand it, the reason that a default parameter value must be given
in the declaration xor the definition of a function is that that prevents
inconsistency between the two (ARM 8.2.6, pp. 144-5).  Here we seem to have
a similar problem, except that it's between two declarations.

What would be the ramifications of prohibiting the specification of a
default value for a parameter in a redefinition of an inherited virtual
function, i.e., requiring that derived classes use the default parameter
values specified in the virtual functions they inherit?

Scott


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