C++ allows you to define a class inside another class, which is called a nested class or a member class. You can also define a template class inside another class, which is called a nested template class or a member template class.

A nested template class is a class template that is declared within the scope of another class template or non-template class. Here's an example:

template <typename T>
class Outer {
public:
    template <typename U>
    class Inner {
    public:
        void doSomething(T t, U u) {
            // ...
        }
    };
};

In this example, we define a template class Outer that contains a nested template class Inner. The Inner class takes a template parameter U and has a member function doSomething that takes arguments of types T and U.

To create an instance of the Inner class, we need to specify both the type of T and the type of U. We can do this using the following syntax:

Outer<int>::Inner<double> inner;
inner.doSomething(42, 3.14);

In this example, we create an instance of the Inner class, which has types int and double for its template parameters. We can then call the doSomething member function, passing 42 and 3.14 as arguments.

Nested template classes are useful when you want to define a template class that is closely related to another class, and that should only be used within the context of the outer class. They can also be used to define helper classes that are used internally by the outer class.

Overall, nested template classes are a powerful tool that can be used to create complex and reusable data structures in C++.