C++ 原生內建的資料型態 ( Primitive Types )
C++ 定義了一些算術用的資料型態 ( Arithmetic Types ) 和一個特殊的資料型態 void。Arithmetic Types : Integer, Character, Boolean, Floating Point, Double Floating Point, Wide Character。
下面為一些資料型態轉換 :
bool b = 42; // b is true
int i = b; // i has value 1
i = 3.14; // i has value 3
double pi = i; // pi has value 3.0
unsigned char c = -1; // assuming 8-bit chars, c has value 255
signed char c2 = 256; // assuming 8-bit chars, the value of c2 is undefined
Initialization
各種初始 :
int units_sold1 = 0;
int units_sold2 = {0};
int units_sold3{0};
int units_sold4(0);
{} 跟 () 的不同 :
long double ld = 3.1415926536;
int a{ld}, b = {ld}; // error: narrowing conversion required
int c(ld), d = ld; // ok: but value will be truncated
Declaration 跟 Definition
為了支援多個檔案的編譯，cpp 區分了 Declaration 跟 Definition。Declaration 代表著跟整個程式宣告一個帶有資料型態的名子，任何檔案都可以去作存取。Definition 則是創造一個物件實體去對應那個帶有資料型態的名子。
如果想要只宣告名子(代表實際物件會被在其他檔案給實現)，則會在宣告時加上 extern，有了 extern 就代表你不能下 Definition。
extern int i; // declares but does not define i
int j; // declares and defines j
extern double pi = 3.1416; // error: ‘pi’ has both ‘extern’ and initializer
Compound Types - References
Reference 等於創造額外的一個名子給一個物件。所以下面第 refVal2, refVal4 會 error，因為你不能給不存在的物件 ( 或 rvalue ) 取名。References must be initialized.
int ival = 1024;
int &refVal = ival; // refVal refers to (is another name for) ival
int &refVal2; // error: a reference must be initialized
int &refVal3 = refVal; // ok: refVal3 is bound to the object to which refVal is bound, i.e., to ival
int &refVal4 = 10; // error: initializer must be an object
Compound Types - Pointers
Pointer為一個 "指向" 物件。類似 References 但卻是一個物件，所以可以不用被 initialized，也可以用來複製、分配。下面的 ＆ 是作為 operator，用來取物件的實際位置，與上面提到用來宣告資料型態的的 ＆ 有著不同的意義。
double dval;
double *pd = &dval; // ok: initializer is the address of a double
double *pd2 = pd; // ok: initializer is a pointer to double
int *pi = pd; // error: types of pi and pd differ
pi = &dval; // error: assigning the address of a double to a pointer to int
下面的 * 作為 operator，放在 pointer 前面等於取其指向的該物件
*p = 0; // * yields the object; we assign a new value to ival through p
std::cout << *p; // prints 0
更多 References & Pointers 範例
double dval;
double *pd = &dval; // ok: initializer is the address of a double
double *pd2 = pd; // ok: initializer is a pointer to double
int *pi = pd; // error: types of pi and pd differ
pi = &dval; // error: assigning the address of a double to a pointer to int
int i = 42;
int &r = i; // & follows a type and is part of a declaration; r is a reference
int *p; // * follows a type and is part of a declaration; p is a pointer
p = &i; // & is used in an expression as the address-of operator
*p = i; // * is used in an expression as the dereference operator
int &r2 = *p; // & is part of the declaration; * is the dereference operator
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