[C++] C++小笔记4--union & Enum

亦凉 2022-09-19 01:47 139阅读 0赞

### 1. union ###

union和struct的结构相同，都可以有多种不同的类型变量，但是用途有区别。union一次只能有一个变量有值。

E.g. struct里面有int, string和double. union里面有int OR string OR double.

In .h
    		union unionId {
    			int id_int;
    			double id_double;
    		};

In .cpp file:
    	unionId ui;
    	ui.id_int = 1;
    	cout << "The value of a union is " << ui.id_int << " double value is " << ui.id_double <<endl;
    	ui.id_double = 12.3;
    	cout << "The value of a union is " << ui.id_int << " string value is " << ui.id_double <<endl;
    	
    Result is:
    	The value of a union is 1 double value is 4.94066e-324
    	The value of a union is -1717986918 string value is 12.3

One use for a union is to save space when a data item can use two or more formats but never simultaneously. For example,

In .h file:
    	struct {
    		string name;
    		int size[4];
    		union id {
    			int id_int;
    			char id_chars[20];
    		} testid;
    	} test;
    	
    In .cpp file:
    	cout << "Enter a or b";
    	cin >> test.name;
    	cout<<endl;
    	if (test.name=="a") {
    		test.testid.id_int = 1;
    	} else {
    		cout << "Enter struct test id in char[] format: ";
    		cin>> test.testid.id_chars;
    	}
    	cout << " The id of test struct is " << test.testid.id_int << "  string format is " << test.testid.id_chars <<endl;
    	
    Result is:

Enter a or b: b
    Enter struct test id in char[] format: afdsga
     The id of test struct is 1935959649  string format is afdsga

一个匿名的union没有名字， 那么它的所有变量就会共享一个地址，只有其中的一个才是当前的。 An anonymous union has no name; in essence, its members become variables that share the same address. Naturally, only one member can be current at a time

struct {
    		string name;
    		int size[4];
    		union {
    			int id_int;
    			char id_chars[20];
    		};
    	} test;

Because the union is anonymous, the id\_int and id\_chars are treated as the variables of the struct test.

test.id_int OR test.id_chars

### 2. Enumeration ###

Enumeration提供了另一种方式来声明const变量。 它的声明方式与struct一样。

By default, enum的第一个元素的值是0， 后续元素在前一个元素的基础上+1。

enum Color { red, green, blue};

1.  color是新定义的一个type;
2.  red, green, blue叫做enumerators. By default, its values start from 0, red=0, green=1, blue=2.

使用Enumeration就可以像声明变量一样。

Color sColor;

#### 赋值方式： ####

Color sColor;
    	sColor = Color(0);  //强制转换
    	sColor = green;     //用enum Color中的成员
    	int iColor = sColor; //用于整数运算
    	cout << "the value of enum is "<< sColor << " and int is "<<iColor<<endl;

Result is:

the value of enum is 1 and int is 1.

*  Enum可以直接将成员赋值给自己类型的变量； E.g. sColor = green;
 *  Enum可以promoted to int type; E.g. int iColor = sColor;
 *  Enum不可以直接用整数赋值, 需要类型转换； E.g.
    
     *  sColor = 0; //not valid
     *  sColor = Color(0);
 *  Enum的变量可以用于运算. E.g.  iColor = 3+sColor;   if(sColor| 1) \{xxx;\}

#### enum的设值 ####

因为enum实际上是const变量的另一种定义方式，我们可以指定enum的元素的值。

*  如果多个元素的值是挨着的。可以用下面enum Values的方式;
 *  如果元素的值是间隔的，可以用Bits or BigStep的方式;

enum Bits {one = 1, two=2, four=4};
    	enum Values {zero, null = 0, pos, positive = 1};
    	enum BigStep {firstStep, bigstep = 100, ThridStep};
    	cout <<"values of Bits:" << one <<" "<<two<<" "<<four<<endl;
    	cout <<"values of Values: " << zero <<" "<<null<<" "<<pos<< " "<<positive<<endl;
    	cout <<"values of BigStep: " << firstStep << " "<< bigstep <<" "<< ThridStep<<endl;
    
    Result is:
    	values of Bits:1 2 4
    	values of Values: 0 0 1 1
    	values of BigStep: 0 100 101