Re: Why need copy constructor to be public here?

In following code:

#include <iostream>
using namespace std;

class Test
{
    public:
        Test(int n) {cout << "ctor" << endl;}
        Test& operator=(const Test& aList) {cout << "assign" << endl; return
*this;}
        operator int() {cout << "operator int"<< endl; return 5;}
              Test(const Test&) {cout << "copy ctor" << endl;}
};

int main()
{
    Test b(3);
    Test a = (int)b;
    return 0;
}

output is:

ctor
operator int
ctor

I can see that copy constructor is not called. Now I change copy
constructor to be private, and recompile g++ gives following errors:

c.cpp: In function `int main()':
c.cpp:12: error: `Test::Test(const Test&)' is private
c.cpp:20: error: within this context
c.cpp:12: error: `Test::Test(const Test&)' is private
c.cpp:20: error: within this context
c.cpp:20: error: initializing temporary from result of
`Test::Test(int)'

So why can't I put copy constructor as private here even it is not used?