C++ namespace相关语法实例分析

#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 = 165;
  cout<<"Method 1: Jerry height: " << jerry::height <<" cm"<<endl;

  //Method 2: using声明
  using jerry::weight;
  weight = 64;
  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 = 2;
  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;
}