18.2. Namespaces

Large programs tend to use independently developed libraries. Such libraries also tend to define a large number of global names, such as classes, functions, and templates. When an application uses libraries from many different vendors, it is almost inevitable that some of these names will clash. Libraries that put names into the global namespace are said to cause namespace pollution.

Traditionally, programmers avoided namespace pollution by using very long names for the global entities they defined. Those names often contained a prefix indicating which library defined the name:

class cplusplus_primer_Query { ... };
string cplusplus_primer_make_plural(size_t, string&);

This solution is far from ideal: It can be cumbersome for programmers to write and read programs that use such long names.

Namespaces provide a much more controlled mechanism for preventing name collisions. Namespaces partition the global namespace. A namespace is a scope. By defining a library’s names inside a namespace, library authors (and users) can avoid the limitations inherent in global names.