Why do I get compilation error in this code?

Question

I have this code and when I compile it I get compilation error.

    #include <iostream>
    #include <string>
    using namespace std;

    struct Date{
      int day;
      int month;
      int year;
    };

  class appointment{
     public:
       appointment(int d,int m,int y,string n);
     private:
       struct Date date;
       string name;

   };

   int main()
   {
     appointment a(22,12,2019,"James"); 
   } 

   appointment::appointment(int d,int m,int y,string n) :
      date.day(d),date.month(m),date.year(y),name(n)
   {

   }

However, when I comment the constructor and use this version of the constructor everything is ok

  appointment::appointment(int d,int m,int y,string n) 
//date.day(d),date.month(m),date.year(y),name(n)
{
  date.day=d;
  date.month=m;
  date.year=y;
  name=n;
}

Answer 1

appointment 's constructor's member-initialiser can only initialise appointment 's own, direct members.

It cannot reach into those members and directly initialise their members.

It has to initialise date , not date 's members.

Now, date itself has no constructor, but it doesn't need one because you can use aggregate initialisation with it:

appointment::appointment(int d,int m,int y,string n)
    : date{d, m, y}
    , name(n)
{}

You did not encounter the problem with your second attempt, because that did not initialise anything! It merely provided a sequence of assignments (as made possible by the public interface of the Date type), and that's fine.

One might argue that this a limitation of the language, but I think it's sensible. You wouldn't want people to only half-initialise your classes. That could lead to inconsistent and unpredictable results.

Answer 2

You can't initialize objects piece by piece like that. Instead, use aggregate initialization to initialize your whole date member all in one go:

appointment::appointment(int d, int m, int y, std::string n)
    : date{d, m, y},
      name{n}
{
}