Run-Time Type ID and the Casting Operators in VS .NET

Drawer QR-Code in VS .NET Run-Time Type ID and the Casting Operators

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tandard C++ contains two features that help support modern, object-oriented programming: run-time type identification (RTTI for short) and a set of four additional casting operators Neither of these were part of the original specification for C++, but both were added to provide enhanced support for run-time polymorphism RTTI allows you to identify the type of an object during the execution of your program The casting operators give you safer, more controlled ways to cast Since one of the casting operators, dynamic_cast, relates directly to RTTI, it makes sense to discuss them in the same chapter

Run-Time Type Identification (RTTI)

Run-time type information may be new to you because it is not found in nonpolymorphic languages, such as C In nonpolymorphic languages there is no need for run-time type information because the type of each object is known at compile time (ie, when the program is written) However, in polymorphic languages such as C++, there can be situations in which the type of an object is unknown at compile time because the precise nature of that object is not determined until the program is executed As explained in 17, C++ implements polymorphism through the use of class hierarchies, virtual functions, and base-class pointers Since base-class pointers may be used to point to objects of the base class or any object derived from that base, it is not always possible to know in advance what type of object will be pointed to by a base pointer at any given moment in time This determination must be made at run time, using run-time type identification To obtain an object's type, use typeid You must include the header <typeinfo> in order to use typeid Its most commonly used form is shown here: typeid(object) Here, object is the object whose type you will be obtaining It may be of any type, including the built-in types and class types that you create typeid returns a reference to an object of type type_info that describes the type of object The type_info class defines the following public members: bool operator==(const type_info &ob); bool operator!=(const type_info &ob); bool before(const type_info &ob); const char *name( ); The overloaded == and != provide for the comparison of types The before( ) function returns true if the invoking object is before the object used as a parameter in collation order (This function is mostly for internal use only Its return value has

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nothing to do with inheritance or class hierarchies) The name( ) function returns a pointer to the name of the type Here is a simple example that uses typeid

// A simple example that uses typeid #include <iostream> #include <typeinfo> using namespace std; class myclass1 { // }; class myclass2 { // }; int main() { int i, j; float f; char *p; myclass1 ob1; myclass2 ob2; cout cout cout cout cout cout cout cout cout cout << << << << << << << << << << "The type of i is: " << typeid(i)name(); endl; "The type of f is: " << typeid(f)name(); endl; "The type of p is: " << typeid(p)name(); endl; "The type of ob1 is: " << typeid(ob1)name(); endl; "The type of ob2 is: " << typeid(ob2)name(); "\n\n";

if(typeid(i) == typeid(j)) cout << "The types of i and j are the same\n"; if(typeid(i) != typeid(f)) cout << "The types of i and f are not the same\n";

if(typeid(ob1) != typeid(ob2)) cout << "ob1 and ob2 are of differing types\n"; return 0; }

The output produced by this program is shown here:

The The The The The type type type type type of of of of of i is: int f is: float p is: char * ob1 is: class myclass1 ob2 is: class myclass2

The types of i and j are the same The types of i and f are not the same ob1 and ob2 are of differing types

The most important use of typeid occurs when it is applied through a pointer of a polymorphic base class In this case, it will automatically return the type of the actual object being pointed to, which may be a base-class object or an object derived from that base (Remember, a base-class pointer can point to objects of the base class or of any class derived from that base) Thus, using typeid, you can determine at run time the type of the object that is being pointed to by a base-class pointer The following program demonstrates this principle

// An example that uses typeid on a polymorphic class hierarchy #include <iostream> #include <typeinfo> using namespace std; class Mammal { public: virtual bool lays_eggs() { return false; } // Mammal is polymorphic // }; class Cat: public Mammal { public: //

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