“Hello，world”
2.1. C++和C语言的关系
C++兼容C语言的绝大多数语法，所以C语言的"Hello World"在C++中也能运行。但是C++有自己的输入输出方式，更现代化、更方便。
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// C语言的Hello World #include <stdio.h> int main() { printf("Hello World\n"); return 0; }代码语言：javascript
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// C++的Hello World #include <iostream> using namespace std; int main() { cout << "Hello World" << endl; return 0; }这里有几个关键点需要解释：
#include <iostream>：C++的输入输出流库，相当于C语言的#include <stdio.h>using namespace std;：使用标准命名空间，让cout、endl等可以直接使用cout：C++的输出流，相当于C语言的printf<<：流插入运算符，用于将数据插入到输出流中endl：换行并刷新缓冲区，相当于C语言的\n
2. 2 为什么C++的输入输出更友好？
C语言的printf和scanf需要手动指定格式，比如printf("%d", a)，而C++的cout可以自动识别变量类型：
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int a = 10; double b = 3.14; cout << a << " " << b << endl; // 输出：10 3.14这就像C++自带了一个"智能助手"，能自动判断你输出的是什么类型，不需要你手动指定格式。
三、命名空间：解决名字冲突的"小助手"
3.1. 为什么需要命名空间？
在C语言中，如果两个文件定义了同名的函数或变量，编译器就会报错，就像两个同学都叫"小明"，老师很难区分谁是谁。
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// 文件1 int rand = 10; // 文件2 int rand = 20; // 编译错误：重复定义C++引入了命名空间（namespace），就像给每个人分配了一个"家庭"，在同一个家庭内可以有同名的人，但不同家庭之间不会冲突。
3.2.命名空间怎么用
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// 定义命名空间 namespace bit { int rand = 10; int Add(int left, int right) { return left + right; } } int main() { // 访问bit命名空间中的rand cout << bit::rand << endl; // 输出：10 // 访问bit命名空间中的Add函数 cout << bit::Add(1, 2) << endl; // 输出：3 return 0; }命名空间的三种方式：
指定命名空间访问（推荐）：
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cout << bit::rand << endl;using将命名空间中某个成员展开：
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using bit::rand; cout << rand << endl;展开命名空间中全部成员（不推荐，可能引起冲突）：
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using namespace bit; cout << rand<<endl;四、C++输入输出：比C语言更智能
1. 为什么C++的输入输出更好？
C语言的输入输出需要手动指定格式，比如scanf("%d", &a)，而C++的cin可以自动识别变量类型：
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int a; doubie b; cout << a << b; //能够自动识别 a是 int 型，b是 double型这就像
2. 为什么需要using namespace std;？
C++标准库中的所有函数和对象都放在std命名空间中，比如cout、cin、endl等。using namespace std;就是把std命名空间中的所有内容"导入"到当前作用域，这样我们就可以直接使用cout、cin等，而不需要写std::cout。
提醒：在实际项目中，我们不建议使用
using namespace std;，因为它可能引起名字冲突。但在学习阶段，为了方便，可以暂时使用。
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