Friend functions and namespaces. Cannot access private member in class

问题

So I have a class inside a foo namespace, which includes a friend function. Now I want the definition of the friend function to be in a different namespace bar so it can be called the way you see below. The error I get is that the private member val cannot be accessed.

Question: Why?

#include <iostream>

namespace foo 
{
    template<typename T>
    class myclass
    {
    private:
        T val;
    public:
        myclass(T v) : val(v) {}

        template<class U>
        friend void myfun(myclass<U>);
    };

    namespace bar 
    {
        template<class U>
        void myfun(myclass<U> a)
        {
            std::cout << a.val;
        }
    } //bar
} //foo

int main()
{
    foo::myclass<int> a(5);
    foo::bar::myfun(a);
}

回答1:

You should declare foo::bar::myfun before the friend declaration and use appropriate namespace qualification (bar::):

namespace foo 
{
    template<typename T>
    class myclass;

    namespace bar 
    {
        template<class U>
        void myfun(myclass<U> a);
    } //bar

    template<typename T>
    class myclass
    {
    private:
        T val;
    public:
        myclass(T v) : val(v) {}

        template<class U>
        friend void bar::myfun(myclass<U>);
    };

} //foo

Otherwise another function called myfun will be declared in the foo namespace by the friend declaration.

来源：https://stackoverflow.com/questions/34212750/friend-functions-and-namespaces-cannot-access-private-member-in-class