Run-time type information


In computer programming, run-time type information or run-time type identification is a feature of the C++ programming language that exposes information about an object's data type at runtime. Run-time type information can apply to simple data types, such as integers and characters, or to generic types. This is a C++ specialization of a more general concept called type introspection. Similar mechanisms are also known in other programming languages, such as Object Pascal.
In the original C++ design, Bjarne Stroustrup did not include run-time type information, because he thought this mechanism was often misused.

Overview

In C++, RTTI can be used to do safe typecasts, using the dynamic_cast<> operator, and to manipulate type information at runtime, using the typeid operator and std::type_info class.
RTTI is available only for classes that are polymorphic, which means they have at least one virtual method. In practice, this is not a limitation because base classes must have a virtual destructor to allow objects of derived classes to perform proper cleanup if they are deleted from a base pointer.
RTTI is optional with some compilers; the programmer can choose at compile time whether to include the functionality. There may be a resource cost to making RTTI available even if a program does not use it.

typeid

The typeid keyword is used to determine the class of an object at run time. It returns a reference to std::type_info object, which exists until the end of the program. The use of typeid, in a non-polymorphic context, is often preferred over dynamic_cast<class_type> in situations where just the class information is needed, because typeid is always a constant-time procedure, whereas dynamic_cast may need to traverse the class derivation lattice of its argument at runtime. Some aspects of the returned object are implementation-defined, such as std::type_info::name, and cannot be relied on across compilers to be consistent.
Objects of class std::bad_typeid are thrown when the expression for typeid is the result of applying the unary * operator on a null pointer. Whether an exception is thrown for other null reference arguments is implementation-dependent. In other words, for the exception to be guaranteed, the expression must take the form typeid where p is any expression resulting in a null pointer.

Example


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class Person ;
class Employee : public Person ;
int main

Output :
Person
Employee
Person*
Employee
Employee

dynamic_cast and Java cast

The dynamic_cast operator in C++ is used for downcasting a reference or pointer to a more specific type in the class hierarchy. Unlike the static_cast, the target of the dynamic_cast must be a pointer or reference to class. Unlike static_cast and C-style typecast, a type safety check is performed at runtime. If the types are not compatible, an exception will be thrown or a null pointer will be returned.
A Java typecast behaves similarly; if the object being cast is not actually an instance of the target type, and cannot be converted to one by a language-defined method, an instance of java.lang.ClassCastException will be thrown.

Example

Suppose some function takes an object of type A as its argument, and wishes to perform some additional operation if the object passed is an instance of B, a subclass of A. This can be accomplished using dynamic_cast as follows.

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  2. include
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using namespace std;
class A ;
class B: public A ;
void MyFunction
int main

Console output:
Method specific for B was invoked
Method specific for B was invoked
Exception std::bad_cast thrown.
Object is not of type B
A similar version of MyFunction can be written with pointers instead of references:

void MyFunction