[书摘]Polymorphism first, RTTI second

RTTI allows you to discover type information from an anonymous base-class reference. Thus, it’s ripe for misuse by the novice, since it might make sense before polymorphic method calls do.

Putting a feature in a base class might mean that, for the benefit of one particular class, all of the other classes derived from that base require some meaningless stub of a method. This makes the interface less clear and annoys those who must override abstract methods when they derive from that base class. For example, consider a class hierarchy representing musical instruments. Suppose you wanted to clear the spit valves of all the appropriate instruments in your orchestra. One option is to put a clearSpitValve( ) method in the base class Instrument, but this is confusing because it implies that Percussion and Electronic instruments also have spit valves. RTTI provides a much more reasonable solution in this case because you can place the method in the specific class (Wind in this case), where it’s appropriate. However, a more appropriate solution is to put a prepareInstrument( ) method in the base class, but you might not see this when you’re first solving the problem and could mistakenly assume that you must use RTTI.

Finally, RTTI will sometimes solve efficiency problems. Suppose your code nicely uses polymorphism, but it turns out that one of your objects reacts to this general purpose code in a horribly inefficient way. You can pick out that type using RTTI and write case-specific code to improve the efficiency. Be wary, however, of programming for efficiency too soon. It’s a seductive trap. It’s best to get the program working first, then decide if it’s running fast enough, and only then should you attack efficiency issues—with a profiler (see Chapter 15).

1）If it is not late-binding, it is not polymorphism.------只有运行时绑定，才是多态。一个很常见的误区是，方法重载并不是多态，因为程序中使用哪个方法在编译的时候就已经确定了，所以方法重载不是多态。
2）尽量使用多态，只有在多态无法解决或者是可以大大提高运行效率的情况下，才使用RTTI。