the use of “union” in all derived classes

It's extremely awsome to realize all design pattern by C，but I'm wondering why there is a union type in each derived class，such as 
```
struct _FindPwdMessage
{
	union
	{
		AbstractMessage;
		AbstractMessage abstractMessage;
	};
};
```
why it not been expressed like 
```
struct _FindPwdMessage
{
	AbstractMessage abstractMessage;
};
```