c++ - 为什么不能在局部类中定义友元函数，而全局类就可以？

Question

#include <iostream>
#include <vector>
#include <string>
using namespace std;
//struct X {
//    friend void f() {}//这里就没问题
//};
int main()
{
    struct X {
        friend void f() {}//error C2689: “f”: 不能在局部类中定义友元函数
    };
}

Answer 1

C and C++ functions cannot define functions inside functions.

Answer 2

The scope of local classes is already very limited. Do you think it is necessary to declare friends? Why is there such a need?