[C++] C++小笔记4--union & Enum

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本文详细介绍了C++中联合体（union）与枚举类型（enumeration）的使用方法及特性。联合体允许不同类型的变量共享同一段内存空间，而枚举则提供了一种定义常量的有效手段。文章通过实例演示了如何使用这两种类型，并解释了它们的工作原理。

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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};

color是新定义的一个type;
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