Boost C++ Application Development Cookbook - Second Edition

Casting polymorphic objects

Imagine that some programmer designed such an awful interface as follows (this is a good example of how interfaces should not be written):

struct object {     virtual ~object() {} };  struct banana: public object {     void eat() const {}     virtual ~banana(){} };  struct penguin: public object {     bool try_to_fly() const {        return false; // penguins do not fly    }    virtual ~penguin(){} };  object* try_produce_banana(); 

Our task is to make a function that eats bananas, and throws exceptions if something different came instead of banana ( try_produce_banana() may return nullptr)so if we dereference a value returned by it without checking we are in danger of dereferencing a null pointer.