Typename

Typename is a keyword in the C++ programming language. It signals to the compiler that a following dependent name should be treated as a type. The keyword is most often encountered in templates, where type names can depend on template parameters.

In templates, typename serves two main roles. First, it can declare a template type parameter, alongside the alternative keyword class. For example, template<typename T> struct Box { /* ... */ }; is equivalent to using class in this context: template<class T> struct Box { /* ... */ }; The two forms are interchangeable for type parameters.

Second, typename is used to disambiguate dependent names that may be either a type or something else. When a dependent name refers to a nested type inside a template parameter, you must prepend it with typename to indicate that it is a type. For example: template<class T> void f(T t) { typename T::value_type v; } Without typename, the compiler cannot determine whether T::value_type is a type or a value.

Typename also appears in conjunction with other constructs, such as alias templates, where it is used to refer to a nested type within a parameterized type: template<class T> using value_type = typename T::value_type;. Outside of templates, typename has no meaning and is not used to declare variables or functions.

Notes and caveats: using typename is specific to C++’s template mechanism and its dependent types. Misplacing it can lead to compile-time errors, often indicating that a dependent name is not a type. The keyword plays a subtle but essential role in template programming and type traits.