TypeId

typeid is a C++ operator used to obtain runtime type information (RTTI). It yields a reference to a const std::type_info object that describes the type of its operand. The operator is declared in the <typeinfo> header and its result is typically used for type comparisons or debugging rather than normal program logic.

There are two forms of typeid. The expression typeid(T) takes a type T and returns the type information for that type. The expression form typeid(expression) takes an expression; if the expression is a glvalue of a polymorphic type (a type with at least one virtual function) and the object has a dynamic type different from its static type, typeid(expression) yields the dynamic type. If the expression is not polymorphic, or if the dynamic type cannot be determined, the static type is used. In practice, typeid on non-polymorphic types yields static type information.

The result of typeid is a const std::type_info&. The type_info class provides name() to obtain a string identifying the type (implementation-defined and potentially mangled), operator== and operator!= for comparisons, and hash_code() (since C++11). It also offers before() to order type_info objects. Since typeid relies on RTTI, RTTI support must be enabled in the compiler; some environments can disable it with a flag.

Usage notes: typeid is most often used to compare types or to print type information for debugging. It is not a substitute for polymorphic behavior and should be used with care in performance-sensitive code. Common alternatives for type discrimination include virtual functions and dynamic_cast.

---