Implicit Conversion to a user-defined type

Question

The following code works fine.

class A
{
   private:
   int _value;
   public:
   class AProxy
   {
     public:
     AProxy(int num):_num(num){}
     int _num;
   };
   A(AProxy x):_value(x._num){}
   int getvalue(){
        return _value;
   }
};

void func(A a)
{
  cout<<"A is initialized with "<<a.getvalue()<<endl;
}

int main()
{
   A a(10);   
   return 0;
}  

a(10) gets converted to a(Aproxy(10))

However, the following code do not work.

class A
{
   private:
   int _value;

   public:
   class AProxy
   {
     class AAProxy
     {
     public:
     AAProxy(int num):_aanum(num){}
     int _aanum;
     };
     public:
     AProxy(AAProxy aa):_num(aa._aanum){}
     int _num;
   };
   A(AProxy x):_value(x._num){}
   int getvalue(){
        return _value;
   }
};

void func(A a)
{
  cout<<"A is initialized with "<<a.getvalue()<<endl;
}

int main()
{
   A a(10);
   return 0;
}

a(10) -> a(Aproxy(AAproxy(10))). So the implicit conversion from int to a user-defined type, happens only once ?

Answer 1

It only looks for direct conversions from type A (what it is) to type B (what it should be). There are way too many ways to convert from A through C to B if it were to do that and the problem becomes unbounded; not to mention that ambiguities are quick to come up that way.

Answer 2

Per the C++ standard, the language will only try to resolve user type disconnects with 1 level of implicit conversion. > 1 level requires explicit conversion.

Answer 3

您将获得最多一个隐式用户定义的转换。