2.6 What is a namespace?

At an informal level, a namespace is the set of all names that have been declared at some point in a program.

Often, programmers will #include a header file in order to use just one or two names decalred in that header. But because the header file actually declares dozens or maybe hunders of other items, all of those names become part of the programmer’s namespace. This effect is sometimes called namespace pollution.

The problem with namespace pollution is that it increases the odds that the programmer might accidentally choose a variable or function a name that is already declared in a header somewhere and has been, unbeknownst to the programmer, found its way into the namespace. The result of such a namespace collision is usually an error message, and often a fairly unintelligable one at that.

Even worse, in large programs a programmer might be trying to use headers from two different commercial libraries. If these two libraries try to use the same name for anything, the resulting namesapce collision would force the programmer to choose between the two libraries.

For these reasons, the C++ standardization committee introduced the C++ namespace as a way to control namespace pollution. A namespace is a “container” of names. Every library is supposed to use its own namespace and only to declare things within that namespace. This includes the C++ standard library, hwere everything is declared within the namesapce std. Declaring things within a namespace is easy:

namespace 
MyCS361Library 
{




 
int 
foo 
();




 
const 
int 
bar 
= 
21;





};
  

1. Put the namespace name in front, connected by “ :: ”. E.g.,

   
   
   
   
   
   
   void 
   baz
   (
   int
   & 
   i
   )
   
   
   
   
   
   {
   
   
   
   
    
   i 
   = 
   i 
   + 
   MyCS361Library
   ::
   bar 
   - 
   MyCS361Library
   ::
   foo
   ();
   
   
   
   
   
   }
       
   
   
   
   
   
   

2. Use a “using declaration” to make a single name from the namespace visible:

   
   
   
   
   
   
   void 
   baz2
   (
   int
   & 
   i
   )
   
   
   
   
   
   {
   
   
   
   
    
   using 
   MyCS361Library
   ::
   bar
   ; 
   
   // 
   makes 
   bar 
   visible
   
   
   
   
    
   i 
   = 
   i 
   + 
   bar 
   - 
   MyCS361Library
   ::
   foo
   ();
   
   
   
   
   
   }
       
   
   
   
   
   
   

3. Use a “using directive” to make everything declared in a namespace visible:

   
   
   
   
   
   
   void 
   baz2
   (
   int
   & 
   i
   )
   
   
   
   
   
   {
   
   
   
   
    
   using 
   namespace 
   MyCS361Library
   ; 
   
   // 
   makes 
   foo 
   and 
   bar 
   visible
   
   
   
   
    
   i 
   = 
   i 
   + 
   bar 
   - 
   foo
   ();
   
   
   
   
   
   }