How to declare a templated struct/class as a friend?

问题

I\'d like to do the following:

template <typename T>
struct foo
{
    template <typename S>
    friend struct foo<S>;

private:
    // ...
};

but my compiler (VC8) chokes on it:

error C3857: \'foo<T>\': multiple template parameter lists are not allowed

I\'d like to have all possible instantiations of template struct foo friends of foo<T> for all T.

How do I make this work ?

EDIT: This

template <typename T>
struct foo
{
    template <typename>
    friend struct foo;

private:
    // ...
};

seems to compile, but is it correct ? Friends and templates have very unnatural syntax.

回答1:

template<typename> friend class foo

this will however make all templates friends to each other. But I think this is what you want?

来源：https://stackoverflow.com/questions/3292795/how-to-declare-a-templated-struct-class-as-a-friend