C++ keyword: typename

 

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Keywords

alignas (C++11) alignof (C++11) and and_eq asm auto (*) bitand bitor bool break case catch char char8_t (C++20) char16_t (C++11) char32_t (C++11) class compl concept (C++20) const consteval (C++20)     constexpr (C++11) constinit (C++20) const_cast continue co_await (C++20) co_return (C++20) co_yield (C++20) decltype (C++11) default delete	do double dynamic_cast else enum explicit export extern false float for friend goto if inline int long mutable namespace new noexcept (C++11)     not not_eq nullptr (C++11) operator or or_eq private protected public register	reinterpret_cast requires (C++20) return short signed sizeof (*) static static_assert (C++11) static_cast struct switch template this thread_local (C++11) throw true try typedef typeid typename union unsigned using virtual void volatile wchar_t while xor xor_eq
Identifiers with special meaning
final (C++11) import (C++20)	module (C++20) override (C++11)

 

The typename keyword is a hint to the compiler that the name following it is a type. It can also be used as a synonym for class keyword in template parameter lists to emphasize that the template can be passed a built-in type or struct. In some places the keyword is optional when the compiler can easily infer that the identifier is a type.

Usage

• In a template declaration, typename can be used as an alternative to class to declare type template parameters and template template parameters(since C++17).
• Inside a declaration or a definition of a template, typename can be used to declare that a dependent qualified name is a type.
• Inside a declaration or a definition of a template,(until C++11) typename can be used before a non-dependent qualified type name. It has no effect in this case.
• Inside a requirements for type requirements (since C++20)
• The type inside the angle brackets of a static_cast, const_cast, dynamic_cast, or reinterpret_cast can use typename
• The type named by an operator new expression can use typename