如何使在某个命名空间中声明的类成为在全局范围内定义的其他类的朋友

Question

I want class A::B to be the friend of C. There is another class with same name in namespace D. How can I do this. In actual hierarchy, different classes may be in different headers.

#include <iostream>
#include <cstdlib>
using namespace std;


class C
{
    friend class A::B;
};

namespace A
{
    class B
    {
    };
}

namespace D
{
    class B
    {
    };
}

int main()
{    
    cout << "Hello, friend!" << endl;
}

Answer 1

You need to forward declare class B (in the correct namespace) before class C :

namespace A {
    class B;
}