Runtime Type Identification in C++

The Runtime Type Identification (RTTI) mechanism remains part of the C++ language and works essentially the same way as in earlier versions. Here's a breakdown with C++23 context:
✅ RTTI in C++23: Still Valid and Active
C++23 continues to support RTTI through:

1. dynamic_cast

   • Used to safely cast a pointer or reference to a base class into a pointer or reference to a derived class.
   • Works only on polymorphic types (i.e., classes with at least one virtual function).
   • At runtime, it checks the actual object type and returns nullptr (for pointers) or throws std::bad_cast (for references) if the cast fails.

   class Base { virtual void foo() {} };
   class Derived : public Base {};
   
   Base* b = new Derived();
   Derived* d = dynamic_cast<Derived*>(b); // Safe downcast
       

2. typeid

   • Returns a const std::type_info& object representing the type of the object or expression.
   • Can be used on both pointers and references.
   • If used on a polymorphic type via a base class pointer or reference, it gives the most-derived type.

   #include <typeinfo>
   #include <iostream>
   
   Base* b = new Derived();
   std::cout << typeid(*b).name(); // Shows "Derived" if RTTI is enabled
       

3. type_info

   • The type returned by typeid, defined in <typeinfo>.
   • Has member functions like name(), before(), and supports comparisons.

🧠 Key Notes for C++23

• RTTI is still enabled by default in most C++ compilers (like GCC, Clang, MSVC).
• RTTI can be disabled using compiler flags (e.g., -fno-rtti for GCC/Clang) — if so, dynamic_cast and typeid for polymorphic types will not work.
• C++23 does not introduce changes to RTTI behavior, but as C++ evolves toward reflection (a future direction), RTTI may coexist or be complemented by newer introspection features.

✅ Conclusion

• The statement you provided is accurate and applicable to C++23.
• RTTI remains a core language feature, particularly for safe polymorphic downcasting and type introspection at runtime.

Runtime Type Identification (RTTI) provides a mechanism for safely determining the type pointed at by a base class pointer at runtime.
RTTI involves

1. dynamic_cast, an operator on a base class pointer
2. typeid, an operator for determining the type of an object
3. type_info, a structure providing runtime information for the associated type

We have already examined dynamic_cast. We will look at the other two components to RTTI in the next lesson.

**Run-Time Type Information (RTTI)** is a C++ feature that allows programs to discover the specific type of an object during execution. This mechanism, while not part of the original C++ design due to concerns about potential misuse, provides a way to perform operations that depend on the dynamic type of an object, a concept known as type introspection. The two primary tools for RTTI in C++ are the `dynamic_cast` operator and the `typeid` operator.
How It Works: The Virtual Function Table's Role
At the core of RTTI's implementation for polymorphic classes (classes with at least one virtual function) lies the **virtual function table (vtable)**. Each object of a polymorphic class contains a hidden pointer, the vpointer, which points to the vtable for its class. This table is a static array of function pointers for all the virtual functions of the class. Crucially for RTTI, the vtable also typically contains a pointer to the `type_info` object associated with that class. The `type_info` object, defined in the `<typeinfo>` header, stores metadata about the type, including its name.
When you use `typeid` on a polymorphic object, the program follows the object's vpointer to its vtable and then retrieves the pointer to the `type_info` object. This allows for checking the exact type of an object at runtime.
The `dynamic_cast` operator also relies on this `type_info` data. When you attempt to downcast a base class pointer to a derived class pointer, `dynamic_cast` uses the RTTI information to traverse the class hierarchy at runtime and determine if the conversion is valid. If the cast is successful, it returns a pointer to the derived object; otherwise, it returns `nullptr`. For reference types, an invalid cast throws a `std::bad_cast` exception.
Performance Overhead: A Necessary Trade-off
Enabling RTTI does introduce some performance and memory overhead.

• Memory Overhead: A small amount of extra memory is required for the type_info object for each polymorphic class.
• Performance Overhead:
  • typeid: For polymorphic types, typeid involves a vtable lookup, which is a very fast operation, comparable to a virtual function call.
  • dynamic_cast: This is generally slower than typeid because it may need to walk the inheritance hierarchy to verify the cast, which can involve several comparisons. The performance impact is more noticeable in complex inheritance structures.

While the overhead for a single operation is often negligible, frequent use of `dynamic_cast` in performance-critical code can become a bottleneck.

Despite the overhead, RTTI is a valuable tool in several scenarios:

1. Safe Downcasting: The most common use case is to safely cast a base class pointer to a derived class pointer when you need to access members specific to the derived class.
2. Debugging and Logging: RTTI can be used to log the specific types of objects being processed, which can be invaluable for debugging complex systems.
3. Serialization and Deserialization: When saving and loading objects, RTTI can help in identifying the correct object type to reconstruct from the stored data.
4. Implementing Reflection-like Features: While C++ doesn't have full reflection capabilities like some other languages, RTTI can be used to build systems that can inspect and manipulate objects based on their runtime type.
5. Plugin Architectures: RTTI can be used to verify the types of objects loaded from dynamic libraries (plugins) to ensure they conform to the expected interfaces.

Criticisms and Alternatives: A Matter of Design
The primary criticism of RTTI revolves around software design. An over-reliance on `dynamic_cast` can be a sign of a flawed class hierarchy or a failure to properly utilize virtual functions. Often, a well-designed system can avoid the need for explicit type checking. Several alternatives to RTTI exist, promoting more robust and maintainable designs:

• Virtual Functions: The cornerstone of polymorphism in C++. By defining a virtual function in the base class and overriding it in derived classes, you can achieve type-specific behavior without needing to know the exact type of the object.
• Visitor Pattern: This design pattern provides a way to add new operations to a class hierarchy without modifying the classes themselves. It uses a "visitor" object that is passed to the elements of a structure, and the element's type determines which method of the visitor is called.
• Custom Type Identification: For performance-critical applications or when RTTI is disabled, developers can implement their own lightweight type identification systems. This often involves adding a type identifier (e.g., an enum) to the base class.

The dynamic_cast<> operation and typeid operator in C++ are part of RTTI. The C++ run-time type information permits performing safe typecasts and manipulate type information at run time. RTTI is available only for classes which 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 function. There may be a resource cost to making RTTI available even if the program does not use it.