Re: The merits of dynamic_cast<>()

From:

Noah Roberts <dont@reply.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 22 Sep 2009 15:57:23 -0700

Message-ID:

<MPG.2522f632f1de9466989691@news.mcleodusa.net>

In article <decedae9-56ee-4ae7-937d-bf45c4699c24
@q35g2000vbi.googlegroups.com>, duggar@alum.mit.edu says...

class A {
public :
   class ErrorIsNotB { } ;
   class ErrorIsNotC { } ;
   int fooA ( ) { return 0 ; }
   int fooB ( ) { throw ErrorIsNotB() ; }
   int fooC ( ) { throw ErrorIsNotC() ; }
} ;

class B : public A {
public :
   int fooB ( ) { return 0 ; }
} ;

class C : public A {
public :
   int fooC ( ) { return 0 ; }
} ;

I think a more intuitive set of objects needs to be hypothesized to show
the problem here:

class Vehicle
{
public:
  virtual void turn(double rad) = 0;
  virtual void forward(double speed) = 0;

  // I put this here because I need to call
  // it on cars that are Vehicle*
  virtual void insert_gas(double amount) { throw "Wot?! 0_o."; }
};

class Car : Vehicle
{
public:
...implement vehicle interface...
  void insert_gas(double amount) { ... }
};

class Bicycle : Vehicle
{
public:
  ...implement turn and forward...
};

Although *syntatically* we could say that the Bicycle appears to be a
Vehicle it actually isn't. Inserting gas into a bicycle is nonsense.

Thus we don't put "insert_gas" into Vehicle because it's convenient. We
take it out of that interface and put it where it belongs, in Car. Only
then will Bicycle be a proper subtype of Vehicle.