Template parameter as a friend

问题

In C++03 the following is illegal, although some compilers support it.

template <class T>
class X
{
    friend T;
};

Has this been legalized in C++11? (Sorry, didn't have time to read the draft myself, just hoping someone knows this)

回答1:

From section §11.3, 3 in N3291:

template <typename T> class R {
  friend T;
};

R<C> rc; // class C is a friend of R<C>
R<int> Ri; // OK: "friend int;" is ignored

So it is legal in C++11.

回答2:

Yes c++0x allows template parameter to be friends.

Well, I happened to remember read it in the draft before but could not find the reference..anyways @Praetorian's answer nailed it.

回答3:

It is illegal in plain C++, but there is a simple workaround

template <class T>
class X
{
    private:
        class Wrapper
        {
            public:
                typedef T Type;
        };
        friend class Wrapper::Type;
};

来源：https://stackoverflow.com/questions/6510041/template-parameter-as-a-friend