Dynamic cast

In computer programming, run-time type information or run-time type identification (RTTI) refers to a C++ mechanism 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 (Delphi).

In the original C++ design, Bjarne Stroustrup did not include run-time type information, because he thought this mechanism was often misused.

C++ RTTI can be used to do safe typecasts, using the dynamic_cast<> operator, and to manipulate type information at run time, using the typeid operator and std::type_info class.

RTTI is available only for classes which 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.

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 a constant-time procedure, whereas dynamic_cast must 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.

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