命名空间和私有静态类成员

Question

Why does this work:

#include "iostream"

class Something {
private:
    static int s_nIDGenerator;
    int m_nID;
    friend int main();
public:
     Something() { m_nID = s_nIDGenerator++; }
     int GetID() const { return m_nID; }
};

int Something::s_nIDGenerator;

int main() {
    Something::s_nIDGenerator = 1;

    Something cFirst;
    Something cSecond;
    Something cThird;

    using namespace std;
    cout << cFirst.GetID() << endl;
    cout << cSecond.GetID() << endl;
    cout << cThird.GetID() << endl;
    return 0;
}

it prints:

1
2
3

And this fail:

#include "iostream"

namespace test {   
    class Something {
    private:
            static int s_nIDGenerator;
            int m_nID;
            friend int main();
    public:
            Something() { m_nID = s_nIDGenerator++; }
            int GetID() const { return m_nID; }
    };
};

int test::Something::s_nIDGenerator;

int main() {
    using namespace test;
    Something::s_nIDGenerator = 1;
    // or test::Something::s_nIDGenerator = 1;  same effect if not using using.

    Something cFirst;
    Something cSecond;
    Something cThird;

    using namespace std;
    cout << cFirst.GetID() << endl;
    cout << cSecond.GetID() << endl;
    cout << cThird.GetID() << endl;
    return 0;
}

With the compiler error message of:

**** Internal Builder is used for build               ****
g++ -O0 -g3 -Wall -c -fmessage-length=0 -o src\tuttest1.o ..\src\tuttest1.cpp
..\src\tuttest1.cpp: In function 'int main()':
..\src\tuttest1.cpp:23:5: error: 'int test::Something::s_nIDGenerator' is private
..\src\tuttest1.cpp:27:13: error: within this context
Build error occurred, build is stopped
Time consumed: 161  ms. 

How do I get the 2nd example to work using the namespace test?

How/Why is the namespace declaration around the object preventing the static member form being accessible?

---

Per my comment to @zmo, here is what I got to work based on his clue:

(comment doesn't have the space or formatting for this, and I had to edit because I couldn't set this an answer.... (what ever it takes.)

#include "iostream"

namespace test {
    class Something {
    private:
        static int s_nIDGenerator;
        int m_nID;
        friend void load(int);
    public:
        Something() { m_nID = s_nIDGenerator++; }
        int GetID() const { return m_nID; }
    };

    int Something::s_nIDGenerator;

    void load (int value) {
       Something::s_nIDGenerator = value;
    } 

};

int main() {
    using namespace test;
    load (1);

    Something cFirst;
    Something cSecond;
    Something cThird;

    using namespace std;
    cout << cFirst.GetID() << endl;
    cout << cSecond.GetID() << endl;
    cout << cThird.GetID() << endl;
    return 0;
}

I am still a little loose as to the "what's up with static members being in a class and a namespace not working?" What's up with this? Why didn't test::Something::s_nIDGenerator work? (still a part of my original question.) So, we are half-answered, so far.

I want to know why this didn't work so I don't walk into this rake again.

Answer 1

Probably because your friend int main() declaration is declaring that the namespace also has a free main() function, while the real main() function is not in the namespace.

To fix it? First declare int main(); before (and outside) namespace test , then friend int ::main() to indicate it's in the global namespace.

For more details, see this question .

Answer 2

well, though I will never recommand you to do like you did in your question, here is how to make your code work "as is" :

#include <iostream>

int main(); // declare main beforehands so it can be seen by Something

namespace test {   
    class Something {
    private:
            static int s_nIDGenerator;
            int m_nID;
            friend int ::main(); // take the main from global namespace
    public:
            Something() { m_nID = s_nIDGenerator++; }
            int GetID() const { return m_nID; }
    };
};

int test::Something::s_nIDGenerator;

int main() {
    using namespace test;
    Something::s_nIDGenerator = 1; // tada that works

    Something cFirst;
    Something cSecond;
    Something cThird;

    using namespace std;
    cout << cFirst.GetID() << endl;
    cout << cSecond.GetID() << endl;
    cout << cThird.GetID() << endl;
    return 0;
}

but here is a wrong use case of a friend function. The solution that seemed to work for you that I suggested (use a function inside your class Something) is far better for readability and understandability.