In all of your programs this semester, you have include the compiler directive using namespace std;. What exactly is a namespace, why do we include this statement?

What's a Namespace?

Since different programmers may work on the same project, it's possible that they will use the same name for two different things (class, function, etc). Namespaces provide a way of grouping the names created by each programmer.

A namespace is a collection of class names, variable names, function names and other name definitions.

It's possible to include more than one namespace in your program. The scope of a namespace is limited by the block of code in which it is used, the same as for variables. However, if the same name is defined in multiple namespaces in the same block, a compiler/linker error will occur.

Namespace Specification

Suppose that we plan to use the function named f which is defined in the namespace Fred. There are three ways to make the function f available to your code.
  1. As we have done this semester, we can insert a using directive into our code using namespace Fred; Not only does this make the function f known to the compiler, it also makes all names in Fred known.

  2. We can specify that we only want to use f from namespace Fred by inserting using Fred::f

  3. We can omit any using directive and always qualify the function name when it's used by writing Fred::f( ) instead of just f( )
Many authorities prefer the 2nd method listed above, inserting all statements like this at the top of your file. This method doesn't include unused names from the namespace which might cause potential conflicts, documents what names are being used from each namespace and doesn't clutter your code by always qualifying your names. The downside is that this list of names must be maintained as new names are used or old ones removed.

Creating a Namespace

To create a namespace of your own, let's call it CMSC202, place code, class definitions, etc. in a namespace grouping as in namespace CMSC202 { // some functions, classes, etc } You can create as many namespace groupings for CMSC202 as you want, (even in different files) and all will be considered part of the same namespace.

We can then insert using namespace CMSC202; into our code to make our names known to the compiler.

More on Namespaces

See section 11.2 of the text for more details on namespaces including the "global namespace" and "unnamed" namespaces.
Dennis Frey
Last modified: Mon Dec 6 12:47:42 EST 2004