c++ - 好友模板函数无法访问私有(private)成员

Question

我有一个关于 this 的问题@GManNickG 编写的代码。

我本来想看看如果我真的明白发生了什么，所以我像这样编辑了 print_binary_helper 的友元函数(原始代码已被注释):

//template <typename U>
//friend print_binary_helper<U> print_binary(U value);
friend print_binary_helper<T> print_binary(T value);

//template <typename U>
//friend std::ostream& operator<<(std::ostream& sink,
//  const print_binary_helper<U> source);
friend std::ostream& operator<<(std::ostream& sink,
    const print_binary_helper<T> source);

//template <typename U>
//friend std::wostream& operator<<(std::wostream& sink,
//  const print_binary_helper<U> source);
friend std::wostream& operator<<(std::wostream& sink,
    const print_binary_helper<T> source);

使用 T 而不是 U，但程序无法编译。有人可以向我解释我做错了什么吗？如果这是可能的，如果是的话，怎么办？

我正在使用 VC++ 11，这是我得到的错误:

1>anything.cpp(68): error C2248: 'print_binary_helper<T>::print_binary_helper' : cannot access private member declared in class 'print_binary_helper<T>'
1>          with
1>          [
1>              T=int
1>          ]
1>          anything.cpp(31) : see declaration of 'print_binary_helper<T>::print_binary_helper'
1>          with
1>          [
1>              T=int
1>          ]
1>          anything.cpp(73) : see reference to function template instantiation 'print_binary_helper<T> print_binary<int>(T)' being compiled
1>          with
1>          [
1>              T=int
1>          ]
1>anything.cpp(68): error C2248: 'print_binary_helper<T>::print_binary_helper' : cannot access private member declared in class 'print_binary_helper<T>'
1>          with
1>          [
1>              T=unsigned __int64
1>          ]
1>          anything.cpp(31) : see declaration of 'print_binary_helper<T>::print_binary_helper'
1>          with
1>          [
1>              T=unsigned __int64
1>          ]
1>          anything.cpp(75) : see reference to function template instantiation 'print_binary_helper<T> print_binary<unsigned __int64>(T)' being compiled
1>          with
1>          [
1>              T=unsigned __int64
1>          ]

Answer 1

template <typename U>
friend print_binary_helper<U> print_binary(U value);

制作一个模板 print_binary 函数 friend 。

friend print_binary_helper<U> print_binary(U value);

创建一个非模板 print_binary 函数 friend 。

两者不同。所以在你的情况下模板函数是不是 friend 并且未定义非模板函数。你不会得到任何错误，因为你没有在任何地方使用非模板 print_binary。

函数是 friend 。所以他们不应该依赖于类的模板参数。它们应该是独立的函数。

---

如果您只想使这些函数的 T 特化成为 print_binary_helper 类的 T 特化的 friend ，您可以转发声明函数和然后像你在类里面所做的那样，稍作修改，将它们专门化。像这样的事情。

template <typename T>
class print_binary_helper;

template <typename T>
std::ostream& operator<<(std::ostream& sink,
                         const print_binary_helper<T> source);

template <typename T>
std::wostream& operator<<(std::wostream& sink,
                          const print_binary_helper<T> source);

template <typename T>
print_binary_helper<T> print_binary(T value);

template <typename T>
class print_binary_helper
{
public:
    static_assert(std::is_integral<T>::value,
                  "Cannot print non-integer in binary.");

    //make only  print_binary<T> a friend to print_binary_helper<T>
    friend print_binary_helper<T> print_binary<>(const T value);
                                //            ^^    

    friend std::ostream& operator<< <>(std::ostream& sink,
                                //  ^^
                                      const print_binary_helper<T> source);

    friend std::wostream& operator<< <>(std::wostream& sink,
                                //   ^^
                                     const print_binary_helper<T> source);

这是一个 Example