本章包含知識點：
1. 常用型別大小及基本規律
2. 常量表達式
3. const和extern關鍵字
4. 標頭檔案相關

1. integeral types（整值型別 指int, char, bool，而整型特指int）

2. • define sizeof(machine word) = W
   • 1 byte = 8 bits
   • 1 word = 4 bytes(typically)

   • 2^16=32768 ， 2^32=2147483648

   • (at least)實際上只規定了最小值，並無嚴格規定

   type	size(byte)
   char	1
   wchar_t	2
   size_t	4
   short	2 or W/2
   int	4(2 at least) or W
   long	4 on 32 and 8 on 64
   float	4
   double	8
   long long	8
   long double	8

3. 注意，if(s<0)s=-s;不一定都能取到s的絕對值——當s是個有符號數且為最小值時，-s還是它自己，因為-s已經超出範圍了，按照規則轉換到範圍內之後恰好還是原來的那個最小負數（max positive + 1 = min negative）。另外，這裡的-

   也不是減號，而是一個一元取反操作符。

4. If the value given to an object is out of its range, then if it’s an unsigned value, it would always be adjusted to be a non-negative number, it gets the remainder of the value modulo the range of that type; if it’s signed value, then usually the same way but not guranteed.

   unsigned
    
    short k1 = 32768 * 2;
   short k2 = -32769;//The range is 32678*2,it's 32678*2-32769
   cout << k1 << ' ' << k2;//0, 32767

5. Literal integer constant：

   024;// octal
   0x14;// hexadecimal
   20;// decimal

   Default type is int or long，但可加上字尾強制轉換：
   
12u;// unsigned
12L;// long
12UL or 12LU;// unsigned long

   【注意！！！】整數字面常量沒有負數！常見的-1其實是一個表示式，取1的相反數，負號是一元取反操作符。-10u其實是4294967286，即10取反後為-10但又被調整到非負範圍內（強制型別轉換髮生在最後）

6. Floating-point literal constant：
   3.14e5 , 0E0 , .001f，大小寫皆可，預設double，字尾f轉單精度，字尾L轉extended precision（極少用）

7. • ‘a’ ： char
   • L’a’：wchar_t
   • 字元可用'\ooo'或'\xddd'八和十六進位制數字表示該字元，如’\115‘(‘M’)

8. Chracter String Literals

   • an array of const char，可以退化為指向常量字元的指標
   • 自動加終止符(null character)
   • wide string literal字首加L

   • 多個由空格類間隔分隔的字串會自動連線成一個，這樣就可以多行輸出一個長句了，但若連線常字串和寬字串，it’s undefined. Putting a backslash as the last character on a line causes the line and the next to be treated as a single line. Any leading spaces or tabs on the subsequent lines are part of the literal.

     cout<<"This is "
           "a " "Hello "  "world\n";
     cout<<"This is a \
           Hello word\n";//More than one whitespace!
     

9. Definitely, we should not rely on machine dependent behaviors, such as assume that the size of int is a fixed and known value like 4.

10. copy-initialization uses the equal(=) symbol, direct-initialization places the initializer in parentheses.

11. 全域性變數不初始化預設為0,' ',NULL...

12. • A definition of a variable allocates storage for the variable and may also initialize it——create it. There must be one and only one definition of a variable in a program.
    • A declaration makes known the type and name of the variable to the program. A definition is also a declaration.
    • Any variable used in more than one file requires declarations that are separate from the variable’s definition.
      

      extern int i;// declares but does not define i,
                   // with a initializer become definition
      int i;//declares and defines i
           // Can't define a variable twice

13. scope分為global, local, statement三種，在底層裡可用同名覆蓋上一層，甚至連型別都能改變。In a word, names can be redefined in an inner scope. But this may be confusing so we should also avoid it.

14. const的意義：

    • readability，從不明意義的常量變為更易讀的變數;
    • maintainability，修改的時候只要在定義處修改即可，方便很多。而且加上const不會被無意中修改掉
    • const變數預設是相對於檔案local，即便你把它寫在全域性位置上。除非定義的時候用extern const int a=8;這樣子，那就相當於可以被其他檔案使用了。non-const variable的話，只要放在檔案的全域性位置上，然後在別的檔案中用extern typeName variableName;即可獲取~

15. extern是一個關鍵字，它告訴編譯器存在著一個變數或者一個函式，如果在當前編譯語句的前面中沒有找到相應的變數或者函式，也會在當前檔案的後面或者其它檔案中尋找該定義。這裡的變數和函式當然是指各個檔案的全域性變數啦，因為它不可能連函式（包括main函式）內部都尋找的。

16. 全域性變數重名的話在各個檔案單獨編譯時不出錯，但在將多個檔案連線在一起的時候，這些變數是互相可見的，亦即同名就會collide（這也是header裡宣告的變數必須加extern的原因。設若不加，則若有多個原始檔包含了該標頭檔案，那這變數的定義就出現了兩次了，而且都是在全域性變數的位置上，link的時候就出錯了；另外，這裡的互相可見是在link階段，並不代表可以直接使用其他檔案的全域性變數，因為這樣你壓根過不了自己檔案的編譯，要用別人的全域性變數還是得用extern。

17. • Reference is only an alias. It must be initialized by an object of the same type when created. And once created, there is no way to rebind it to another object.
    • const reference is a reference that may refer to a const object, which is not possible for a non-const reference. But it can also refer to a non-const object. What’s more, it can be initialized to an rvalue or an object of different but related(即可轉換的) type.(實際上是構建了一個臨時的相應型別的變數讓reference指向該變數，由於read only所以可行)
      

      int val = 3;
      const int cVal = 4;
      int &a = val;
      int &b = cVal;//error
      const int &c = val;
      const int &d = cVal;
      c = 5;//error
      int &e = 8;//error
      const int &f = 33;//ok
      double dVal = 3.14;
      const int &g = dVal;//ok, because read-only, same as
                          //int temp=dVal;const int &g=temp;

18. typedef 定義型別的同義詞，如

    typedef double wages;
    wages hourly,weekly;

19. enum open_mode {input = 2, output, append = 3, other};

    The value assigned to enum should be a constant expression, which can be evaluated by compiler at compile time. enumerator is const. But an enum object is not const.

20. When we define a class, we start by defining its interface——the operations that the class will provide. From these operations we can then determine what data the class will require to accomplish its tasks and whether it will need to define any functions to support the implementation.

21. 定義class記得要加分號，報錯的cpp檔案可能是因為它包含的標頭檔案少了分號，記得檢查。

22. class 成員預設private，struct成員預設public。class這個關鍵字還用於定義模板引數，除此之外無差別。

23. Headers normally contains class definition, extern varibale declaration, function declaration. Two benefits:

    1. Consistency, all files are guaranteed to use the same declaration for a given entity;
    2. Only the header needs to be updated when changes are needed.

    A header should logically belong together because it needs time to compile. And headers are for declarations, not definitions. Definitions should NOT occur in a header. e.g. extern int val = 10; double val;//wrong, both are definitions

    However, there are 3 exceptions when we can use definitions: class, const object whose value is known at compile time, inline functions. (const預設是local的，所以可以。如果初始化用到 constant expression，很多時候不存在記憶體消耗而是直接替換了；如果用函式來初始化的話，那就得把它放到原始檔中防止函式被反覆呼叫了，加上extern也可以共享)

24. Header guard

    
    #ifndef HEADER_H
    
    
    #define HEADER_H
    
    //blablabla
    
    
    #endif
    

Reference : C++ Primer 4th edition（評註版）