Constructor, Class, union, struct

Question

I have some doubts I would like to clear regarding class, struct and union. I know class and struct are considered object. I would like to check whether union is consider object too. Whereby, I can have a constructor inside?

union ABC
{
  ABC() {};
  int x;
}A, B, C

Since I have object name. I could do this?

Union ABC obj1(); //Call constructor?
A.x;  //Variable accessing the member

??? Secondly, what if i typedef union. What does ABC mean? Can I do this

union ABC obj1();
A.x;


typedef union ABC
{
  ABC() {};
  int x;
}A, B, C

Answer 1

1. The difference between a struct and a class is that by default members in struct are public, but private in class; otherwise, they are functional equivalent. A union can have member functions (including constructors and destructors), but not virtual functions. A union shall not have base classes. A union shall not be used as a base class.

2. Union ABC obj1(); //Call constructor?

   No, This does not call a constructor

   union ABC obj1() means you declare a function with name obj1 and returns an object of union ABC

3. You have syntax errors in both cases, you have to do the following:

    union ABC { ABC() {}; int x; }A, B, C; //You cannot miss this ; 

4. You can do:

    typedef union ABC ABCUnion; then ABCUnion A, B,C; 

   It is the same as

    union ABC A, B,C;