c++类的继承的基本例程。定义了“动物”类，然后定义了“猫”类并继承动物的特征（猫属于动物），然后定义了“老虎”类并继承猫的特征（老虎属于猫）。简单易懂，适合小白琢磨。(The basic routine of the inheritance of the c++ class. It defines the "animal" class, then defines the "cat" class and inherits the characteristics of the animal (the cat belongs to the animal), and then defines the "tiger" class and inherits the characteristics of the cat (the tiger belongs to the cat). Easy to understand, suitable for small white polish.)