explicit initialization的新知

对于水母精华区里多次讲到的T t = v;的实际过程，这里(叫做explicit initialization)有很明确的解释：

其中一个之前没有注意到的问题是，尽管编译器可以将2步优化为1步，但要求copy ctor必须为public的，否则就会失败。

Even if, as in the above example, the compiler does not generate a call to the copy constructor, it must be available. This is illustrated in the following program (available as test-2.C) which does not compile. 

#include    

    class T {

        public:

        T(int i): data_(i) { std::cerr << "T::T(int)" <<>

        private:

        T(const T&) { std::cerr << "T::T(const T&)" <<>

         int data_;

     };

    int main(int, char**) {

        T t1(3); // OK

        T t2 = 3; // Error: T::T(const T&) private.

     }

For the above program, the compiler issues the following error message

test-2.C: In function `int main(int, char**)':  

test-2.C:6: `T::T(const T&)' is private  

test-2.C:13: within this context  

test-2.C:13: initializing temporary from result of `T::T(int)'