How do I define a templated function outside the class when the class itself is templated?

Question

I've tried:

template <typename T>
struct Class1
{
    template <typename U>
    int func(U u);
};


template <typename U>
int Class1::func(U u)
{

}

It doesn't work.

I've also tried:

template <typename T>
struct Class1
{
    template <typename U>
    int func();
};


template <typename T, typename U>
int Class1<T>::func(U u)
{

}

But neither work.

Answer 1

As mentioned in the comments, it's usually recommended to implement templated methods inline.

However - if you insist on separating the declaration from definition, you can do it in the following way:

template <typename T>
struct Class1
{
    template <typename U>
    int func(U u);
};

template <typename T>
template <typename U>
int Class1<T>::func(U u) 
{
    return 0;
}

Live demo

A side note:
To answer your question in the comment: this is not what is called template template, but rather a simple method template inside a class template.
Template template parameters are generally parameters of a template, which are themselves templates instead of being of normal types.
For more info about template template parameters see here and here.