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The copier is broken << | Home | >> A big unit on Object Oriented Design

Problem 22: I want to know what kind of Book I have

2017-10-31 (happy halloween!)

BOO! -Brad Lushman

For simplicity: Assume the old book hiearchy

  • Book

    • Text
    • Comic

  • C-style Casting

    • (type) expr
    • Forces expr to be treated as type type
    • Ex. C++ int *p; int q = (int) p;
    • Very easy to be a source of error
    • Very difficult to search for

The C++ casting operators - 4 operators

  • static_cast: for conversion with well-defined semantics
    • Ex. 
      void f(int a); 
void f(double d);
int x;
f(static_cast<double>(x));
    • Ex. 
      // superclass pointer to a subclass pointer
Book *b = new Text { ... };
Text *t = static_cast<Text *>(b);
    • You do this if you're 100% sure that pointer is pointing to a Text, otherwise it is not safe
  • reinterpret_cast: for casts without well-defined semantics
    • Unsafe, implementation-dependent (therefore unportable)
    • Ex. 
      Book *b = new Book { ... };
int *p = reinterpret_cast<int *>(b);
  • const_cast: for adding/removing const
    • The only C++ cast that can "cast away const"
    • If item is still in read-only memory, it's still read only
    • Ex. 
      void g(Book &b);    // Assuming we know g won't change b 
void f(const Book &b) {
    g(const_cast<Book &>(b));
}
  • dynamic_cast: Book *pb = ...
    • What if we don't know whether pb points to a Text?
    • static_cast is not safe 
      Text *pt = dynamic_cast<Text*>(pb);
    • If *pb is a Text or a subclass of Text, cast succeeds. pt points at the object, else pt = nullptr. 
      if (pt) { ... pt->getTopic() ... }
else ... // Not a Text
    • Ex. 
      void whatIsIt(Book *pb) {    
    if (dynamic_cast<Text*>(pb)) cout << "Text";
    else if (dynamic_cast<Comic*>(pb)) cout << "Comic";
    else cout << "Book";
}
      • Not good style, what happens when you create a new Book type?
    • Dynamic reference casting 
      Book *pb = ____;
Text &t = dynamic_cast<Text &>(*pb);
if (*pb is a Text) // ok
else // rase std::bad_cast
    • Note: dynamic casting works by accessing an objects Run-Time Type Information (RTTI), this is stored in the vtable for the class
      • This means we can only dynamic_cast on objects that have at least one virtual method
    • Dynamic reference casting offers a possible solution to take polymorphic assignment problem: 
      Text &Text::operator=(Book other) { // Virtual
    Text &textother = dynamic_cast<Text &>(other);  // Throws if other is not a Text
    if (this == &textOther) return *this;
    Book::operator=(std::move(textother));
    topic = std::move(textother.topic);
    return this; 
}

The copier is broken << | Home | >> A big unit on Object Oriented Design