C++ explicit constructor/copy constructor note

C++:explict 作用显示声明构造函数只能被显示调用从而阻止编译器的隐式转换,类似只能用()显示调用，而不能=或者隐式调用

 #include <iostream>
 #include <vector>
 #include <string>
 #include <thread>

 class Demo
 {
 private:
     int a;
 public:
     explicit Demo()
             : a()
     {
         std::cout << "默认构造函数" << std::endl;
     }

     ~Demo()
     {
         std::cout<<"析构函数"<<std::endl;
     }

     explicit Demo(const Demo &other)
     {
         a = other.a;
         std::cout << "拷贝构造函数" << std::endl;
     }

     Demo &operator=(const Demo &other)
     {
         if (&other == this)
             return *this;
         a = other.a;
         std::cout << "拷贝赋值构造函数" << std::endl;
         return *this;
     }

 };

 void test(Demo& T)
 {

 }

 int main()
 {
     Demo c;            //explicit constructor;
     Demo D(c);          //explicit copy constructor

     //test(D)        //this is implicit,error;

     return ;
 }