c++ - How to use friend keyword for template class

Question

lets say that I have 2 template classes, A and B. If I want to make B a friend of A, what would I say ?

class<template T>
class A
{
public:
friend class B<T>; // ???


};

class<template T>
class B
{

};

Answer 1

要使用一个符号，它必须被声明或定义，这在模板中是一样的。您需要转发声明模板B。此外，您声明模板类的语法（class<template T>）无效，它应该是template <class T>.

这应该有效：

template <typename T>  // typename can be replaced with class 
class B;

template <typename T>
class A
{
public:
friend class B<T>;  
};

template <typename T>
class B
{

};