Friend funcitons of template classes

Question

I want to return a nested class object using a friend function

template <typename T>
class X{
public:
    class Y{
    public:
        int y;
    }test;
public:
    X(){
        test.y=10;
    }

    template <typename U>
    friend Y fun(X<U>);
};


template <typename T>
X<T>::Y fun(X<T> x){
    return x.test;
}

But I get an error saying

need 'typename' before 'X::Y' because 'X' is a dependent scope

What is wrong?

Answer 1

You need to do literally what the error says: put typename before X<T>::Y :

template <typename T>
typename X<T>::Y fun(X<T> x){
    return x.test;
}

Because the meaning of X<T>::Y is type-dependent on T , the compiler can't know in general whether X<T>::Y refers to a typename or a variable. In such circumstances the rule is that if you want it to be typename, you must say so explicitly with the typename keyword.

Answer 2

You have to write

template <typename T>
typename X<T>::Y fun(X<T> x){
    return x.test;
}

becouse compliler can't resolve if Y is filed or type.