External Polymorphism and Internal Polymorphism

起因

1、在阅读 stackoverflow Type erasure techniques 时，它的comment中，有这样的描述:

Side note: the pattern used i.a. by Boost.Any is called the external polymorphism pattern – wmamrak Dec 4

其中提及了"external polymorphism"，于是我Google了一下，检索结果中包含 wiki.c2 External Polymorphism ，这篇文章总结地不错，通过其中的内容，我发现我之前很多的做法其实都可以归为 external polymorphism，显然我们使用 external polymorphism 的目的是为了实现 custom polymorphism，即自定义polymorphism。我又联想到了之前总结的custom virtual table，显然它也是为了实现 custom polymorphism；我又联想到了之前阅读的visitor pattern中实现multiple dispatch，其中是明确反对custom polymorphism、external polymorphism的，因此这就是导致了我编写本文。

正文

Google了一下，发现下面这篇文章的介绍是比较好的。

1、agnihotris External Polymorphism (Adapter Design pattern) vs InternalPolymorphism

Internal Polymorphism :- When you call a function based on its dynamic type of objects by using virtual table or vtable . The only methods declared as virtual are polymorphic in c++.

In internal polymorphism function bindings are part of class definition of the object .

NOTE: internal polymorphism是指C++ programming language内置支持的

External polymorphism :- Here functions and their bindings are defined independently of a class.

This allows classes which are not related by inheritance and/or have no virtual methods to be treated polymorphically. Thus unrelated classes can be treated in common manner by s/w that uses them.

NOTE: external polymorphism其实是custom polymorphism，是由programmer自己创建的

This pattern capitalizes on both c++ features and on other basic patterns (Adapter or Decorator) to give appearance of polymorphic behavior of otherwise unrelated class.

2、External-Polymorphism

参见 wiki.c2-External-Polymorphism 章节。