C++中关键字explicit的作用

class Circle 
{ 
public: 
    Circle(double r) : R(r) {} 
    Circle(int x, int y = 0) : X(x), Y(y) {} 
    Circle(const Circle& c) : R(c.R), X(c.X), Y(c.Y) {} 
private: 
    double R; 
    int    X; 
    int    Y; 
}; 

 int _tmain(int argc, _TCHAR* argv[]) 
 { 
  //发生隐式类型转换 
  //编译器会将它变成如下代码 
  //tmp = Circle(1.23) 
  //Circle A(tmp); 
   //tmp.~Circle(); 
   Circle A = 1.23;  
   //注意是int型的，调用的是Circle(int x, int y = 0) 
  //它虽然有2个参数，但后一个有默认值，任然能发生隐式转换 
   Circle B = 123; 
  //这个算隐式调用了拷贝构造函数 
   Circle C = A; 
      
   return 0; 
} 

class Circle 
{ 
public: 
    explicit Circle(double r) : R(r) {} 
    explicit Circle(int x, int y = 0) : X(x), Y(y) {} 
    explicit Circle(const Circle& c) : R(c.R), X(c.X), Y(c.Y) {} 
private: 
    double R; 
    int    X; 
    int    Y; 
}; 
 
 int _tmain(int argc, _TCHAR* argv[]) 
{ 
//一下3句，都会报错 
 //Circle A = 1.23;  
 //Circle B = 123; 
  //Circle C = A; 
   
//只能用显示的方式调用了 
//未给拷贝构造函数加explicit之前可以这样 
    Circle A = Circle(1.23); 
    Circle B = Circle(123); 
    Circle C = A; 
 
 //给拷贝构造函数加了explicit后只能这样了 
    Circle A(1.23); 
    Circle B(123); 
    Circle C(A); 
    return 0; 
}