Can you explain what the 'typename' keyword is in C++ and how it is used in templates?

In C++, the keyword typename is used in templates to indicate that the following identifier is a type name, not, for example, a value or a member.

This is important when the compiler cannot unambiguously determine what the expression represents, especially when working with dependent names — names that depend on template parameters.

Example of using typename:

template <typename T>
void func() {
    typename T::value_type x; // indicating that value_type is a type
}

Without typename, the compiler may produce an error because it is not sure that T::value_type is a type.

typename is also used when declaring templates:

template <typename T>
class MyClass {};

Here, typename and class are interchangeable, but typename is preferred for template type parameters.

In summary, typename helps the compiler correctly interpret type names in templates.