Typetraits

Typetraits are a programming concept used to expose and manipulate properties of types at compile time. They serve as a form of meta-information about types, allowing a program to reason about a type’s characteristics without executing code from that type. Typetraits are commonly implemented as template-based or type-level constructs that map a type to another type or to a compile-time constant.

A typical typetraits design uses a primary template that provides a default result, along with specialized templates that override the result for specific categories of types. The result is often a boolean value or a type such as a true_type or false_type, and may also include related information like removing qualifiers or determining a type’s underlying category. This pattern enables compile-time decision making and is frequently used in conjunction with enablement mechanisms or constraints.

In practice, typetraits enable conditional compilation, overload resolution, and template specialization. For example, a trait might indicate whether a type is a pointer, an integral type, or a const-qualified type, and a program can then select an appropriate implementation based on that information. Modern languages provide built-in facilities for these checks; in C++, type traits are centralized in a standard library that includes facilities such as is_pointer, remove_cv, and enable_if, while newer language features like concepts offer an alternative way to express constraints.

Typetraits also appear in other languages with different forms. In Rust, trait bounds and associated types provide similar capabilities for compile-time specialization, while Haskell uses type classes to constrain operations by type properties. Across languages, typetraits contribute to generic programming by making type information explicit and usable at compile time.

See also: type traits, template metaprogramming, type system.