[C++] namespace相关语法

本段测试代码包括如下内容：

(1) 如何访问namespace中声明的名称；
(2) namespace导致的相关冲突；
(3) namespace可嵌套；
(4) 可以在namespace中使用using声明和using编译命令；
(5) 未命名的namespace：其作用域为定义该namespace所在的声明区域。C++推荐用来替代static定义静态变量。

 #include <iostream>

 using namespace std;

 namespace jerry{

     int height;

     int weight;

     void showHeight();

     string name;

 }

 //

 namespace jerry{

     void showHeight()

     {

         cout<<"Method 3: Jerry height: "<<height<<" kg"<<endl;

     }

 }

 namespace elements

 {

    namespace fire

    {

        int flame;

        using namespace jerry; //(4) can use 'using' in namespace define

        using std::cout;

    }

    float water;

 }

 //(5) no name namespace

 //其作用域为定义时所在的声明域，可用来替换static变量，这是C++标准推荐的行为

 namespace {

     string data;

 }

 void testFun();

 int main()

 {

     cout<<"This code is to test namespace"<<endl;

     /*not allowed to define namespace in code segment

     //Error

     namespace jerry{

           int height;

           int weight;

     }

     */

     //(1) To access the data in namespace

     //Method 1: 作用域解析符

     jerry::height = ;

     cout<<"Method 1: Jerry height: " << jerry::height <<" cm"<<endl;

     //Method 2: using声明

     using jerry::weight;

     weight = ;

     cout<<"Method 2: Jerry weight: " << weight<<" kg"<<endl;

     //Method 3: using编译指令：All the define data in namespace jerry can be access.

     using namespace jerry;

     showHeight();

     //(2) about name conflict

     {

         jerry::name = "Jerry";

         string name = "Tom";

         //using jerry::name; Error

         cout << "name: "<<name<<endl;

         /*

         This method will lead conflict with locall parameter

         using jerry::name;

         cout << "name: "<<name<<endl;

         */

         cout << "name: "<<jerry::name<<endl;

         using namespace jerry;

         //局部变量会覆盖jerry命名空间的name定义

         cout << "name: "<<name<<endl;

     }

     //(3) namespace can nest

     elements::fire::flame = ;

     using namespace elements::fire;

     //(5) no name namespace

     //其作用域为定义时所在的声明域，可用来替换static变量，这是C++标准推荐的行为

     data = "hello";

     cout<<"No name namespace: data: " << data <<endl;

     testFun();

 }

 void testFun()

 {

    /*not allowed to define namespace in code segment

    //Error

    namespace jerry{

          int height;

          int weight;

    }

    */

     //(5) no name namespace

     data = "hello in function";

     cout<<"No name namespace: data: " << data <<endl;

 }

运行结果为：