synthesis obj=_obj

之前收藏了一個解釋，感覺不是太清晰：

這兒有個解釋就不錯：

如果做了x=_x的處理：
@synthesize happiness = _happiness
-(void) setHappiness:(int)happiness{
...
_happiness=happiness;
...
}

如果沒有=_:
@synthesize happiness
-(void) setHappiness:(int)happiness{
...
happiness=happiness//這裏發生混淆
...
}