class variables and class instance variables in Ruby

First, in ruby1.8, class variables are shared by all subclasses and parent classes, and you can look at the following code:

class　IntelligentLife
　@@home_planet　=　nil

　def　 self.home_planet
　　@@home_planet
　end
　def　self.home_planet=(x)
　　 @@home_planet　=　x
　end
　#...　
end
class　Terran　<　IntelligentLife
　@@home_planet　=　"Earth"
end

class　Martian　<　IntelligentLife
　 @@home_planet　=　"Mars"
end

p　IntelligentLife.home_planet
p　 Terran.home_planet
p　Martian.home_planet

You can see that the results are the same. This is because the class variables of the parent class are shared by the entire inheritance system.

Here, if we want to get the results we need, we're going to use class instance variables, because class instance variables are strictly per-class. Rather than being shared by the entire inheritance system.

class　IntelligentLife
　@home_planet　=　nil

　class　<<　self
　　attr_accessor　:home_planet
　end
end
class　Terran　<　IntelligentLife
　@home_planet　=　"Earth"
end

class　Martian　<　IntelligentLife
　 @home_planet　=　"Mars"
end

p　IntelligentLife.home_planet
p　 Terran.home_planet
p　Martian.home_planet