關于模板Default拷貝構造函數與模板Copy拷貝構造函數（- - 我也不知道該不該這么稱呼。。）

#include <iostream>
using namespace std;

template<typename _Ty>
class Test
{
    public:
        Test(){}
        
        Test(const Test& _t)
        {
            cout<<"Default"<<endl;
        }

        template<typename _Uy>
        Test(const Test<_Uy>& _t)
        {
            cout<<"Copy"<<endl;
        }
};

int main()
{
    Test<int> t;
    Test<int> d(t);
    Test<double> f(t);
    return 0;
}

 
編譯錯誤：

--------------------Configuration: x - Win32 Release--------------------
Compiling...
x.cpp
D:\書籍資料\C++Test\c++作業\asdf\x.cpp(16) : error C2535: '__thiscall Test<_Uy>::Test<_Uy>(const class Test<_Uy> &)' : member function already defined or declared
        D:\書籍資料\C++Test\c++作業\asdf\x.cpp(10) : see declaration of 'Test<_Ty>::Test<_Ty>'
        D:\書籍資料\C++Test\c++作業\asdf\x.cpp(20) : see reference to class template instantiation 'Test<_Ty>' being compiled
D:\書籍資料\C++Test\c++作業\asdf\x.cpp(25) : error C2668: 'Test<int>::Test<int>' : ambiguous call to overloaded function
Error executing cl.exe.
Creating browse info file...

x.exe - 2 error(s), 0 warning(s)



看下面的代碼：只是將兩個函數位置調換一下就正確了- -；

#include <iostream>
using namespace std;

template<typename _Ty>
class Test
{
    public:
        Test(){}
        
        template<typename _Uy>
        Test(const Test<_Uy>& _t)
        {
            cout<<"Copy"<<endl;
        }

        Test(const Test& _t)
        {
            cout<<"Default"<<endl;
        }

};

int main()
{
    Test<int> t;
    Test<int> d(t);
    Test<double> f(t);
    return 0;
}