具有更多模板参数的模板友元类

Question

我有一个带有三个模板参数的类模板Obj1

template < class A, class B, class C >
class Obj1
{
      // some implementation
};

和带有两个模板参数的第二类模板Obj2,

 template < class A, class B >
    class Obj2
    {
          // some implementation
    };

所以我的问题如下:

我想让类Obj1成为Obj2类的朋友,前两个模板参数具有相同的值,但我不知道如何编写它的确切语法,起初我试过这种方式

template < class A, class B>
class Obj2
{
    template< class C>
    friend class Obj1<A,B,C>;
};

但它没有编译,所以如果可以,请求帮助我.

Answer 1

您的问题的答案在这里：/sf/answers/102122821/ “您只能与类模板的所有实例或特定的完整专业化‘成为朋友’。”

所以，这是允许的：

template < class A, class B, class C >
class Obj1
{
      // some implementation
};

template < class A, class B>
class Obj2
{
    public:
    template <class C, class D, class E> friend class Obj1; ///All instances
};

template < class A, class B>
class Obj3
{
    public:
    friend class Obj1<A,B,int>; ///One specific instantiation, possibly depending from your own templated parameters
    friend class Obj1<A,B,A>; ///One specific instantiation, possibly depending from your own templated parameters
    friend class Obj1<char,float,int>; ///One specific instantiation, possibly depending from your own templated parameters
};