typeidT

TypeidT is a term used in discussions of runtime type information to denote a type identity object associated with a compile-time type parameter T. It represents the identity of the type itself rather than any value of that type, enabling programs to reason about types at runtime without requiring instances.

Representation and access often involve a type information object such as TypeInfo, Class, or a type_descriptor. Access is typically provided by a language construct or a generic utility, for example typeidT<T> or getTypeInfo<T>(). The resulting object is usually comparable by identity, hashable, and may expose metadata such as the type name, size, qualifiers, or other attributes.

Usage patterns include dynamic dispatch based on type, registration for serialization, reflection systems, and debugging tools. Type identity supports type-safe downcasting, generic dispatch tables, and type-based configuration without constructing concrete objects. It also underpins scenarios like generic serialization schemas and runtime type checks in libraries that support reflection.

Examples are usually presented in pseudocode since the exact syntax varies by language. A typical pattern is obtaining info for a type and comparing it to info for another type, or using a type identity as a key in a registry.

History and variants note that real-world languages implement type information differently: C++ uses the typeid operator and std::type_info; Java uses Class<T>; .NET uses Type. Some languages expose the concept as a pattern or library feature rather than a built-in keyword.

Notes emphasize that the exact semantics depend on the language and implementation. Accessing type identities may incur runtime cost, and researchers often emphasize caching and cross-language compatibility in design discussions.