1.1.8.1 namespace basic knowledge

• basic namespace style:
  1. If you develop a library of functions or classes, put them in a namesapce, just like std:: namespace in STL.
  2. Don’t put more in global-scope, Learn to using namespace instead of global.
  3. Don’t put your own class into namesapce std;
  4. Usually, namespace should be your project name, you can add company name in front of it if you like.
• You can use :: to specify global variable name or function if you have same name in you local scope. Or use Base:: to specify Base scope name if you have same name in derived class.
• Namespaces can be located at the global level or inside other namespaces. They can’t be placed in a block.. So it has external linkage by default, that is to say it can be accessed by multi translation unit(files).
• There are three kinds of namespace usages.
  using sp::name;   //1) using declaration 
  using namespace sp;   //2) using directive 
  sp::name   //3) specific refer it 
   
  namespace ns{ 
  Int zy; 
  } 
  using namespace ns; 
   
  Int main(){ 
  Int zy = 0; //it will hide ns::zy 
  Cin>>zy; //read into local zy 
  }
• For implement code, there are four methods to put it into namespace. Prefer method 3 and method 4.
  //a.h 
  namespace Yan{ 
   Class Foo{ 
       void mem_fun(); 
   }; 
  } 
   
  //a.cpp 
  //mehtod 1. then use name, using declaration 
  using Yan::Foo; 
  void Foo::mem_fun(){.......} 
   
  //method 2.  using directive 
  //It’s BAD 
  using namespace Yan; 
  void Foo::mem_fun(){.........} 
   
  // method 3. 
  //Good style 
  void Yan::Foo::mem_fun(){.........} 
   
  //method 4. 
   //Also good, when you have a lot of function need to be define, 
   //compared with method3, save your typing. 
  namespace Yan{ 
     void Foo::mem_fun(){......} 
  }
• About using directive usage
  1. Don’t use using directive in any .h file, because It will pollute all the .cpp file which include this head file.
  2. Less use using directive in any .cpp file, you should using scope-resolution or using decaring more. It will avoid polluting namespace.
     #include <iostream> 
     using std::cout; 
     using std::endl; 
      
     Int main(){ 
     cout<<"Hello␣world"<<endl; 
     }
  3. If you have to use using directory in your .cpp file, put it after all the include files.
• unnamed namespace just like static to specify it to local file scope. At the same time, make anonymous namespace as small as possible. see C++ primer p492
  // a.cpp file 
  static int count; 
   
  // A better method to use namesapce. 
  namespace{ 
  int count; 
  }