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What is runtime type information?

Runtime type information (RTTI), also known as runtime type identification (RTI), is a feature of several programming languages (such as C++, Object Pascal, and Ada) that makes data about an object's data type available at runtime. It is possible for runtime type information to be made available to all kinds or just to those types that have it explicitly (as is the case with Ada). A more broad idea called type introspection is specialized into runtime type knowledge.

Runtime type information wasn't a part of the original C++ design because Bjarne Stroustrup believed it was frequently abused.

In brief, Runtime type information, or RTTI in C++, is a mechanism that makes information about an object's data type available at runtime and is only available for classes that include at least one virtual function. It enables program execution to determine the type of an item.

What are runtime casts?

The easiest method to determine an object's runtime type using a pointer or reference is to use a runtime cast, which verifies that the cast is legal. This is very useful if we need to cast a pointer from one base class to another. Casting an object is typically necessary when working with the hierarchy of classes that determines their inheritance. The casting comes in two varieties:

  1. Upcasting - when a base class pointer is used to treat a pointer or reference to a derived class object.
  2. Downcasting - when a reference or pointer to a base class is changed to a pointer to a derived class.

Using "dynamic cast": In a hierarchy of inheritance, it is used to cast a base class pointer down to a child class. If casting is successful, it returns a pointer of the converted type. If we attempt to cast an invalid type, such as an object pointer that isn't of the type of the intended subclass, it fails.


Through the use of the dynamic cast> operator in C++, safe typecasts may be performed, and the typeid operator and std::type info class can be used to modify type information in real-time. Safe type casts can be carried out in Object Pascal using the as an operator. The is operator can be used to determine the class to which an object belongs, and classes from the RTTI unit can be used to alter type information at runtime (i.e. classes: TRttiContext, TRttiInstanceType, etc.). In Ada, objects with tagged types also keep track of a type tag that allows runtime type identification. If an object is of a certain type and can be properly converted to it, it can be tested at runtime with the in operator. Only polymorphic classes-that is, classes that include at least one virtual method-can use RTTI. In reality, this is not a restriction since base classes are required to have virtual destructors so that objects of derived classes can execute correct cleanup if they are destroyed from a base pointer.

Some compilers feature flags that make RTTI inoperable. These flags are very helpful when designing applications for systems with limited memory because they may minimize the overall size of the application.

What is C++ typeid?

An object's class can be ascertained at runtime using the typeid keyword. Upon completion of the program, it returns a pointer to the std::type info object. In non-polymorphic contexts where only the class information is required, typeid is frequently favoured over dynamic cast class type> because typeid is always a constant-time process. In contrast, a dynamic cast may need to navigate the class derivation lattice of its input at runtime. std::type info::name(), for example, is implementation-defined and cannot be trusted to remain consistent between compilers.

When the unary * operator is used on a null pointer to create the typeid expression, objects of class std::bad typeid are thrown. Implementation-specific factors determine whether exceptions are raised for additional invalid reference arguments. To put it another way, the expression must have the form typeid(*p), where p is any expression that yields a null pointer.

What is C++ dynamic cast and java cast?

A reference or pointer can be downcast to a more precise type in the class hierarchy in C++ using the dynamic cast operator. Unlike the static cast, the dynamic cast's target must be a pointer or reference to a class. A type safety check is carried out at runtime instead of static cast and C-style typecast, which do their type checks during compilation. When dealing with references, an exception will be thrown, or a null pointer will be returned if the types are incompatible (when dealing with pointers).

Similar to this, a Java typecast will throw a java. Lang.ClassCastException instance if the object being cast is not actually an instance of the target type and cannot be transformed to one by a language-defined method.

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