[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：