# OOAD Lab 1.5, 2 (03/02)
###### tags: `OOAD`

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[TOC]

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## 1.5. Pointer and References
- *"C 的 pointer 是 "indirect addressing mode" （間接定址）的高階語言實作"* ^[CYP]^
- *"Activation record"*:
- An activation record (AR) is a private block of memory associated with an invocation of a procedure. [^[LINK]^](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjKovPd2br9AhWbfd4KHU5UA1cQFnoECBgQAw&url=https%3A%2F%2Fwww.rose-hulman.edu%2Fclass%2Fcsse%2Fcsse404%2Fschedule%2Fday17%2FProcedures.pdf&usg=AOvVaw2q3DaYur9oAsF3a2thFJFy)
- 一個 activation record (stack frame) 儲存程式的：[^[LINK]^](https://liquid0118.pixnet.net/blog/post/48494862)
- 參數與本地變數
- 執行結果(回傳值)
- 返回位址(return address)
- 其他環境資訊
- *"How to swap two variables in C? (pass by address)"*

    void swap(char* a, char* b)
    {
        char t;
        t = (*a);
        (*a) = (*b);
        (*b) = t;
    }

    int main()
    {
        char x, y;
        swap_c(&x, &y);
    }

- "C++'s pass by reference?"

    void swap(char& a, char& b)
    {
        char t;
        t = a;
        a = b;
        b = t;
    }

    int main()
    {
        char x, y;
        swap(x, y);
    }

-

    int main()
    {
        int& a;
    }

    $ g++ a.cpp
    a.cpp: In function ‘int main(int, char**)’:
    a.cpp:3:10: error: ‘a’ declared as reference but not initialized
         int& a;
              ^

- "Java's pass by reference?"

    class T
    {
        int v;
    }

    char c;  // A char-type variable
    do(c);   //   pass by value
    T a;     // A reference variable to T
    a = new T();
    do(a);   //   pass by reference

- "How to swap two (primitive) variables in pass-by-reference in Java?"
- The default wrapper classes in Java is **immutable**, so assigning will init new objects [^[LINK]^](https://stackoverflow.com/q/5560176)
- That is, the passed `Integer` object originally in the ref var in the swapping function will be replaced with newly inited `Integer` object
- ---> one way: manually declare mutable wrapper classes [^[LINK]^](https://stackoverflow.com/a/4319581)

    class IntRef
    {
        public int v;
    }
    
    void swap(IntRef a, IntRef b)
    {
        int t;
        t = a.v;
        a.v = b.v;
        b.v = t;
    }

    main()
    {
        IntRef x = new IntRef();
        IntRef y = new IntRef();
        x.v = 10;
        y.v = 25;
        
        swap(x, y);
    }

## 2. Inheritance
- **All the following is C++!!!**
- `public`:
- publicly derived subtypes of the base class will be able to directly access these variables.
- `protected`:
- only the base class could directly access them
- `private`:
- any class or part of code could access them
- these would defeat a key goal of object-oriented design: **encapsulation** data within a class and exposing the data only through the public interface
- All constructors, including copy constructors, destructors aer not inherited
- inheritance: extending additional behaviors in OO system development
- **Basic principle of inheritance**:
- The base class:
- contains common members and methods used by the subclasses
- The subtypes:
- are more specialized (特異化) then the base class
- **(Cons/des call)**:
- the base class part of objects is always constructed first, destroyed last
- the subclass part of objects is always constructed last, destroyed first
- **每一個 subclass 要負責建構與清除自己的特異化的的部分**
- in default, subclasses use the default constructor of the base class (take no argument)
- Add the constructor of the base class into the **member initialization list**:

        class Pet
        {
        public:
            Pet() : weight(1), food("Pet Chow") {}
            Pet(int w) : weight(w), food("Pet Chow") {}
            Pet(int w, string f) : weight(w), food(f) {}
        protected:
            int weight;
            string food;
        };

        class Rat : public Pet
        {
        public:
            Rat() {}
            Rat(int w) : Pet(w) {}
            Rat(int w, string f) : Pet(w, f) {}
        };

- **Override**:
- A derived class can override the methods of its base class(es)
- The method in the derived class must have the same same **signature** as the base class method to override
- **Overloading**
- many methods of the same name with different signatures are created
- $\ne$ polymorphism
- **Overriding a single one of the overloads will hide the rest!**
- it is better to override all the overloads
- **Composition** (contain other objects as members) $\leftrightarrow$ **Inheritance**
- **The Principle of Inheritance**
1. A *subclass* is a *base class* (recursively)
A *base class* is not necessary a subclass (recursively)
2. *subclass*'s behaviors must be specialized and distinguished from *base class*'s
- **Inheritance without polymorphism is not object-oriented**
- **When you use a subclass to override a base class's method. C++ will use the current type to determine the method**:

    class pet
    {
        void speak() {cout << "Growl" << endl;}
    }
    class pet : public pet
    {
        void speak() {cout << "meow" << endl;}
    }
    
    main()
    {
        pet insect;
        cat pussy;
        pet* nose = (pet*) new cat();
        
        insect.speak();
        pussy.speak();
        ((pet) pussy).speak();
        nose->speak();
    }

    pet constructor  // `insect`
    pet constructor  // `pussy`
    cat constructor  // ...
    pet constructor  // `nose`
    cat constructor  // ...
    Growl            // `insect.speak()`
    meow             // `pussy.speak()`
    Growl            // `((pet) pussy).speak()` => call pet's copy cons first
    pet destructor   // ... call pet's des
    Growl            // `nose->speak()`
    cat destructor   // `pussy`
    pet destructor   // ...
    pet destructor   // `insect`