# OOAD Lab 2.6, 3 (03/09)
###### tags: `OOAD`

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## 2.6. Why You Should Not Abuse Inheritance over Composition
- **Inheritance** vs. **Composition**
- when you try to find answer for OOAD problem it is always about **FUTURE**
- Bad of "Abuse Inheritance over Composition"
1. **Inherit all the method and interfaces of vector**
- all of them can be override?
- **A class only carry the methods needed**
2. **Polymorphism freedom can kick your ass**
3. **Copy-paste program**
- when **"has-a" or composition relationship changed**, you cannot reuse most of the code
- need to do copy-paste program

## 3. Polymorphism
- **多型：就是當你可以用基底 class 的指標或參考來進行處理時，卻可以依照其 subtype 來進行不一樣的事情**（其實多形是很難用解釋的）
- **C++ 中任何想要被繼承的 class overridden 的 method 都要宣告成 virtual**
- **一個function 一旦宣告成virtual，則在繼承的class 中也是 virtual**。但是可以忽略不寫，不過建議要寫
- **多型只對 pointer, reference（a base class points to a subclass object）, 以及宣告為 virtual 的 function 有效**
- **如果一個 class 有任何一個 method 宣告為 virtual 則 destructor 也應該宣告為 virtual** ，這為了確保正確的 destructor 做正確的清除
- Why not declare all methods as virtual?
- This could be done, but **involves additional overhead.**
- **Object slicing**:
- when **pass-by-value (copy)** from subtype to base type
- **only base type part of a subclass instance gets copied**
- **4 basic element of OOP**
- object & message
- inheritance
- encapsulation
- dynamic binding (via polymorphism)
- **polymorphism? vs. overloading**\
- **polymorphism**：函示在執行的時候，面對不同的型別或物件，能有自動相對應的操作及功能
- **overloading**：多出來的功能，是在於可以真正的以一段同樣的程式碼對不同的 資料型別或物件進行操作
- 在物件導向的語言中，**polymorphism (多型） 是inheritance, message passing 所自然結合產生的結果**：
- **switch patterns**
- 耦合性過高
- **---> function pointers**
- `int (*f)(int, int);`, `int (*f[])(int,int);`
- **---> dynamic binding（動態連結）**
- binding of compiler and linker (logical address)
- compilation time binding
- linking time binding
- **indirect call**
- dynamic linking library (DLL)
- external reference table: `ExTbl[index] => run time address
- **virtual function table**
- Dynamic binding via polymorphism
- **virtual function table**
- **if there is a virtual function, a pointer `vptr` (per instance) to a virtual function table (per class) is generated**
- **Virtual Base Class (Abstract class)**
- In C++, **class that has member functions are pure virtual function**
- e.g. `virtual void foo() = 0;`
- **沒有 instance，不能 new**
- 用多重繼承，**那麼只要這個 class 可能有兩個以上的 derived class 則你應該在繼承他的時候在前面加上 virtual**
- **只有繼承沒有多型，不是物件導向**
- 核心程式與 subtype 無關