RunTimeTypeIdentification.pptVIP

Run-Time Type Identification Jim Fawcett CSE687 – Object Oriented Design Spring 2007 Typeid The function: const typeinfo typeid(arg)returns a const typeinfo object when passed an object, reference, or pointer. Typeinfo:class type_info {  public:  virtual ~type_info();  int operator==(const type_info rhs) const;  int operator!=(const type_info rhs) const;  int before(const type_info rhs) const;  const char* name() const;  const char* raw_name() const;  private: ... }; Examples of typeid() use class foo { … };typeid(foo).name() returns “foo” foo *ptr;typeid(ptr).name() returns “foo*”typeid(*ptr).name() returns “foo” class derived : public base { … };typeid(base).before(typeid(derived)) returns true  Shape *sp = new circle;typeid(shape) == typeid(*sp) returns falsetypeid(shape).before(typeid(*sp)) returns truetypeid(sp).name() returns “circle*”typeid(*sp).name() returns “circle” dynamic_cast…(…) Dynamic casts support safe down-casting (casting down an inheritance hierarchy):Line* lptr = new line;circle* cptr = new circle;rectangle* rptr = new rectangle;shape* array[3] = { lptr; cptr, rptr };for(int i=0; i3; i++) if(dynamic_castcircle*(array[I])) cout “circle” endl; else cout “non-circle” endl; Caution Typeid and dynamic_cast information is carried in a class’s virtual function pointer table and is intended to be used only with polymorphic classes, e.g., those with at least one virtual function. Typeid works for non-polymorphic classes, but returns only static type info, e.g., based on the static pointer type, not on the type of the object pointed to. You must enable run-time type information (RTTI) in your project settings (C/C++ tab, C++ language category). Your program will crash if you use dynamic_cast or RTTI and forget to do this. Enabling RTTI End of Presentation * On by default