(C++学习笔记一)命名空间 常量 字符串_namespace 里定义常量

3.1.有关于命名空间 namespace :

3.1.1. 简述：

• 1. 在C++语言编写的程序中，变量和函数等的作用范围是有一定限制的。比如，在函数体中定义的一个临时变量就不可以在函数体外使用。为了解决变量和函数等的作用范围，在C++语言中引入了名空间的概念，并增加了关键字namespace和using.
• 2.在一个名空间中可以定义一组变量和函数，这些变量和函数的作用范围一致，可以将这些变量和函数称为这个名空间的成员。
• 3.通过名空间，可以在同一个文件中使用相同的变量名或函数名，只要它们属于不同的名空间。另外，名空间可以使得代码操作具有相同名字但属于不同库的变量。而且，名空间也可以提高C语言与C++语言的兼容性。

3.1.2.本体

1.using namespace std;//C++标准名称空间

• 未引用标准名称空间时需在关键字前添加 std::

// using namespace std;//C++标准名称空间
 std::cout <<" a = "<< a <<  std::endl;
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注意:这是一个C++标准名称空间,如果将之注释掉,很多C++关键字就会出错!

• 引用时

using namespace std;//C++标准名称空间
cout <<" a = "<< a << endl;
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2.常规应用

#include<iostream>

using namespace std;//C++标准名称空间

int a = 10;

namespace ns//类似于C语言结构体
{
	int a = 2;
	char s = 'f';
	int b = 9;
}

int main()
{
    cout <<" a = "<< a << endl;
	cout << ns::a << endl;
	cout << ns::s << endl;
	system("pause");
	return 0;
}
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3.2.system("pause"); :

• 在C++中一般在main函数中的return之前添加system(“pause”);这样就可以看清楚输出的结果，pause会输出"press any key to continue. . ."。
  `

3.3.常量

C++定义常量两种方式:

3.3.1. #define宏常量 ： #define 常量名 常量值

• 通常在文件上方定义，表示一个常量,不可修改.

3.3.2. const修饰的变量 ：const 数据类型 常量名 = 常量值

• 通常在变量定义前加关键字const，修饰该变量为常量，不可修改.

#define day 7//宏定义
const int month = 12;

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3.4. 字符串定义

char site1[7] = {'R', 'U', 'N', 'O', 'O', 'B'};
char site2[] = "RUNOOB";
string site3 = "runoob";
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3.5. 总结

#include<iostream>
using namespace std;//C++标准名称空间

void name(int nt);//声明函数 
void swap1(int* p1, int* p2);//输入两数指针,并使用解指针对两者数值进行交换

#define stc 10

int a = 10,day = 365;
int b = 0,c = 0;

namespace ns//类似于结构体
{
	int a = 11;
	char s = 'f';
	int b = 9;
	namespace tr//命名空间嵌套
	{
		int a = 12;
	}
}

struct score//成绩结构体 嵌套
{
	int SA;
	int SB;
};

struct staff//员工结构体
{
	string name;//姓名
	int age = 1;//年龄  //若不初始化将会有警告
	struct score sc = {1,1};//若不初始化将会有警告
};


char s[10] = { 'q','w','e','t' };//C风格字符串
char str1[10] = "RUNOOB";//C风格字符串
string str2[] = { "good" };//输出信息不对❌
string str3= "hello world";//C++风格字符串，需要加入头文件 string

bool asa = true;//bool 可直接赋值:true/false 1/0 当赋值不等于0或1时默认为1

int main()
{
	const int dc = 13;//const 修饰常量
//	double dou1;
	int arr[10] = {1,2,3,4,5,6,7,8,9,10};
	int* arr_zz = &arr[4];
	int* arr_zz3 = &arr[3];
	int* p = arr;

	staff S1;//定义结构体变量
	staff S2 = { "MO",18,{1,2} };//定义结构体变量
	staff* S2_zz = &S2;

	cout << "a = " << a     << endl;
	cout << day    << " 天" << endl;
	cout <<  s     << endl;

	cout << "名称空间" << endl;
	cout << ns:: a    << endl;
	cout << ns::s     << endl;
	cout << ns::tr::a << endl;

	cout << "字符串" << endl;
	cout << "s = " << s << endl;
	cout << "\r\nstr1 = "  << str1 << endl;
	cout << "\r\nstr2 = " << str2 << endl;
	cout << "\r\nstr3 = " << str3 << endl;
	cout << "\r\nsizeof(s) = " << sizeof(s) << "\r\nsizeof(str1) = " << sizeof(str1) << endl;
	cout << "\r\nsizeof(str2) =" << sizeof(str2) << endl;

	asa = false;
	cout << "bool型变量"  << endl;
	cout << "asa = " << asa  << endl;
	cout << "sizeof(asa) = " << sizeof(asa) << "\r\n" << endl;

//	cout << "输入浮点型变量:" << endl;
//	cin >> dou1;
//	cout << "dou1 = " << dou1 << endl;
//	cout << "sizeof(dou1) = " << sizeof(dou1) << "\r\n" << endl;

	a = 0;
	a += 2;//a = a + 2;
	cout << "加等 \r\na = 0; a += 2;a = " << a << "\r\n" << endl;
	// +=,*=,-=,%=,/=;  变量 += 常量;  


	a = 10;
	b = 14;
	cout << "比较符" << endl;
	cout << (a > b) << "\r\n" << endl;//真假
	//比较符 == , != , < , > , <= , >= . 

	b = 12;
	c = (a < b) ? 1 : 0;
	cout << "三目运算符(a=10; b=12)\r\n" << "a < b ?  c = " << c << "\r\n" << endl;
	c = (a > b) ? 1 : 0;
	cout << "a > b ?  c = " << c << "\r\n" << endl;
	cout << "a > b ? " << endl;
	a > b ? cout << "true" << endl : cout << "false" << endl;//这里有一个需要注意的地方,注意语句结束的地点.
	cout << "\r\n" << endl;

	cout << "输出实验\r\n\r\n";
	for (a = 0; a < 10; a++)
	{
		for (b = 0; b < 10; b++)
		{
			cout << "*" << " ";//如果添加 <<endl 的话会执行换行
		}
		cout << endl;//换行,移位
	}
	cout << endl;//换行,移位

	//continue与break的区别:
	//continue: 结束本次循环,不执行本次循环余下程序.
	//break   : 结束整个循环.

	for (int a = 0; a < 100; a++)
	{
		if (a % 2 == 0)
		{
			continue;
		}
		cout << a << endl;
	}
	cout << endl;

	c = (sizeof(arr) / sizeof(int))-1;//
	cout << "arr[10] = {";
	for (; c > -1; c--)
	{
		cout << arr[9-c] << ",";
	}
	cout << "};\r\n";
	cout << "数组实验: \r\n" << "\r\n1.获取数组所占内存空间\r\n";
	cout << "sizeof(arr) = " << sizeof(arr) << endl;
	cout << "2.获取数组首地址\r\n" << "&arr = " << &arr << endl;//错误方法
	cout << "(int)&arr[0] = " << (int)&arr[0] << endl;
	cout << "(int)&arr[1] = " << (int)&arr[1] << endl;

	name(100);

	cout << endl << "指针实验" << endl;
//	int* arr_zz = &arr[0];
//	int* a_zz = &a;
	cout << "打印地址(arr[4]):" << "&arr[4] = " << &arr[4] << "    arr_zz = " << arr_zz << endl;
	cout << "打印地址(arr[3]:" << "&arr[3] = " << arr_zz3 << endl;
	cout << "*arr_zz= " << *arr_zz << "//`*`又称为解引用" << endl;// 通过* 操作指针变量指向的内存
	cout << "打印数组arr的首地址:" << "arr = " << arr << "   p = arr = " << p << "   &arr[0] = " << &arr[0] << endl;
	cout <<"指针各个类型所占内存大小:" << endl << "int*" <<sizeof(arr_zz) << endl;
	cout << "sizeof(char*) = " << sizeof(char*) << endl;
	cout << "sizeof(float*) = " << sizeof(float*) << endl;
	cout << "sizeof(double*) = " << sizeof(double*) << endl << endl;
	cout << "数组p = arr指针自加实验" << endl;
	for (a = 0; a < 10; a++)
	{
		cout << "a = " << a << endl
			<< "地址: " << p << endl
			<< "解地址:" << *p << endl;
		p++;
	}
	cout << "输入两数指针, 并使用解指针对两者数值进行交换:" << endl;
	a = 178;
	b = 14;
	cout << "a = " << a << "    b = " << b << endl;
	swap1(&a,&b);
	cout << "a = " << a << "    b = " << b << endl;
	cout << endl << endl;

	cout << "Struct Experiment" << endl;

	cout << "name S1  :" << S1.name << endl
		<< "    " << S1.age << endl
		<< "    " << S1.sc.SA << endl
		<< "    " << S1.sc.SB << endl;

	cout << "name S2  :" << S2.name << endl
		<< "    " << S2.age << endl
		<< "    " << S2.sc.SA << endl
		<< "    " << S2.sc.SB << endl;

	S1.name = "N1";
	S1.age = 23;
	S1.sc.SA = 250;
	S1.sc.SB = 50;
	S2_zz->name = "N2";
	S2_zz->age = 22;
	S2_zz->sc.SA = 22;
	S2_zz->sc.SB = 22;
	cout << "`.`赋值 name S1  : " << endl << S1.name << endl
		<< "    " <<S1.age << endl
		<< "    " << S1.sc.SA << endl
		<< "    " << S1.sc.SB << endl;
	cout << "`->`赋值 name S2  : " << endl << S2.name << endl
		<< "    " << S2.age << endl
		<< "    " << S2.sc.SA << endl
		<< "    " << S2.sc.SB << endl;
	cout << "staff* S2_zz = &S2 = " << S2_zz << endl;


	system("pause");
	return 0;
}

void name(int nt)
{
	int we = 0;
	we = ns::a + nt;
	cout << "\r\n调用函数 name :" << endl;
	cout << "we = ns::a + nt = " << we << "\r\nns::a = " << ns::a << "\r\nnt = " << nt << endl;
}

void swap1(int* p1, int* p2)
{
	int temp = *p1;
	*p1 = *p2;
	*p2 = temp;
}
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